There are thousands of games where NPCs are hidden from the player until he gets to a certain point. Each enemy is a programmed object. He requires a small piece of allocated memory inside RAM. So, the game stores his useful values like health, 3D position coordinates, speed vector, aiming point, AI stuff and so on.
To provide convenient and smooth gameplay, game developers use this trick and this operation is executed during a level loading. When you come to a certain point, enemies just teleport from OoB inside a predefined area (behind walls and edges near you, for example).
If you have ever player such games as S.T.A.L.K.E.R, you'd find that there are no enemies spawned OoB waiting utill the game decides. Instead of it, the game allocates the memory dynamically. So you often face "freezes".
In short, it's a common fact.
TASing is like making a film: only the best takes are shown in the final movie.
