There are two competing interpretations vying for control of the "Wandering Monster" concept, although it seems like most people are unaware that there are even two interpretations.
One interpretation says that wandering monster rolls represent the inhabitants of the dungeon occasionally leaving their lairs to do other things.
One interpretation says that wandering monster rolls represent transient populations.
Those who follow the first interpretation believe that any monster that doesn't fit the list of established occupants is an example of bad design; the author of the dungeon didn't think about how the dungeon would actually work. Those who follow the second interpretation, on the other hand, think of wandering monster tables written with the first interpretation in mind as just another table, perhaps lacking in variety, but perfectly justifiable.
Both in theory are based on the idea that dungeons aren't static. Things change. However, the first interpretation is not sustainable, because if all the monsters on the wandering monster table must be from the existing dungeon population, the list should thin out and eventually there won't be any more wandering monsters; the dungeon becomes static over time.
The thing is: the fact that the adventurers entered the dungeon means other things should be able to enter, too.