One of the gurus explained that behind the loading screen a few moments of game time takes place during which your sims will be placed in a suitable activity. This was to create the feeling of living sims or something like that. If I can find the link to his explanation I'll edit this post to include it. It was in one of the old tech threads.
*edited:
Quote from simgurumax: "Very quickly, I'll say that our persistence (save) model works very differently than TS3. On TS3, we saved the exact game state right down to the contents of memory of the entire simulation. This was cool, because it meant when you loaded the game was exactly as you had left it. Unfortunately, this lead to many problems such as long load times and, worse, save games that would crash or get corrupted and be unrecoverable.
On TS4, we do a softer save, which saves all the interesting data about your game like where all the Sims are and all their stats. When we load a game, we put the Sims back where they were and restore their state. Then we run autonomy to find them an interaction and process a little bit of time to let them get to the point where they are running that interaction. During this small processing time, we put the game into a mode where all routes happen instantaneously to make it so we don't have to let too much time pass to get Sims into position.
All of this happens behind the loading screen.
The end result is you lose a little bit of time when you save and load - somewhere on the order of 5 sim minutes. Saving and loading can also fix some issues now because it does sort of "reset" the whole simulation and any weird cruft that may have built up."
link to the post:
forums.thesims.com/en_US/discussion/782859/simulation-tech-talk/p9
