In Which Gygax Gets Disenchanted with Wandering Monsters

Here's the standard rule for wandering monsters in OD&D Vol-3 (1974), p. 10; underlined emphasis by me:

Wandering Monsters: At the end of every turn the referee will roll a six-sided die to see if a "wandering monster" has been encountered. A roll of 6 indicates a wandering monster has appeared. The direction of appearance is determined by random number generation considering the number of possible entries. Distance and surprise are decided in the usual manner. The kind of monster is determined on the table below... 

Recall that in these rules, one exploratory turn is meant to be 10 minutes. So that's quite a few wandering monsters; we expect one every hour of in-game time at that rate. Now let's fast forward to the AD&D DMG (1979), p. 9, in the first section of "Introduction":

The final word, then, is the game. Read how and why the system is as if is, follow the parameters, and then cut portions as needed to maintain excitement. For example, the rules call for wandering monsters, but these can be not only irritating - if not deadly - but the appearance of such con actually spoil a game by interfering with an orderly expedition You have set up an area full of clever tricks and traps, populated it with well-thought-out creature complexes, given clues about it to pique players’ interest, and the group has worked hard to supply themselves with everything by way of information and equipment they will need to face and overcome the imagined perils. They are gathered together and eager to spend an enjoyable evening playing their favorite game, with the expectation of going to a new, strange area and doing their best to triumph. They are willing to accept the hazards of the dice, be it loss of items, wounding, insanity, disease, death, as long as the process is exciting. But lo!, everytime you throw the "monster die" a wandering nasty is indicated, and the party’s strength is spent trying to fight their way into the area. Spells expended, battered and wounded, the characters trek back to their base. Expectations have been dashed, and probably interest too, by random chance. Rather than spoil such an otherwise enjoyable time, omit the wandering monsters indicated by the die. No, don’t allow the party to kill them easily or escape unnaturally, for that goes contrary to the major precepts of the game. Wandering monsters, however, are included for two reasons, as is explained in the section about them. If a party deserves to have these beasties inflicted upon them, that is another matter, but in the example above it is assumed that they are doing everything possible to travel quickly and quietly to their planned destination. If your work as a DM has been sufficient, the players will have all they can handle upon arrival, so let them get there, give them a chance. The game is the thing, and certain rules can be distorted or disregarded altogether in favor of play.

In summary: an extended a harangue about what a bad idea randomly-generated wandering monsters are. Note the passage references the fact that, "Wandering monsters, however, are included for two reasons, as is explained in the section about them" -- but as far as I can tell, there isn't any section in the book which gives a standard process for checking for wandering monsters (nor any explanation of "two reasons" for them).

This is one of many cases in the transition from OD&D to AD&D in which it's easy to recreate Gary's brain saying, "I'm pretty sure I wrote a rule for that somewhere, right?", with the answer being, "Yes, back in OD&D". Recall the "presumed axiom" understood by Gygax & co., as shared by Frank Mentzer last year:

Presumed Axiom: 1e rules set should expand upon, and not directly contradict, 0e rules.

Anyway, what can we deduce about the rule for AD&D dungeon wandering monsters? Despite the preceding, and without any explicit written rule section in the DMG, looking at a parenthetical aside in the example of play it seems that a change has indeed been made, on p. 98:

(Here, as about 3 turns have elapsed, the DM rolls a d6 to see if a 'wandering monster' appears; the resulting 5 indicates none.)

That is; the checks for wandering monsters have been reduced by a factor of 3. Over the course of a 4-hour adventuring session (say), instead of expecting 4 wandering monsters encounters , now we would only expect around 1. It's a little odd that Gygax didn't call out this change clearly as a rule; perhaps he felt somehow constrained by the "presumed axiom" that it prevented him from doing so.

Interestingly, the earlier Holmes Basic D&D rules (1977) feature the same rule, on p. 10:

At the end of each three turns the Dungeon Master can roll a die to see if a wandering monster has come down the corridor. A roll of 6 means that something has come "strolling" along.

Zenopus Archives informs us that this rule is unchanged between Holmes' initial draft manuscript, and Gygax's later editorial pass. So who initiated this revision? Did Gygax somehow inform Holmes about it, or did Holmes invent it and prompted Gygax to follow suit, or something else?

(Side note: Commentator Chris reminds me that the DMG Random Dungeon Generation has a 1-in-20 chance of a wandering monster per periodic check on Appendix A: Table I. If one roll is made per turn, then that's again roughly equivalent to the 1-in-6 chance every 20/6 ≈ 3 turns.)

Personal opinion: This aspect of Gygax's curating of the rules, as seen in the long DMG p. 9 warning -- "here is the rule, but the rule is bad, so it should be disregarded" -- is probably my least favorite of all gestures that he makes. If we find from experience that a rule is not working satisfactorily, then fix it until it does. Spending time and space making excuses for it, or saying that good DMs can be expected to compensate for it, is not productive. The "presumed axiom" perhaps inculcated too much conservatism in what could have been an opportunity for smart edits in other places from more play experience.