I get it if the terrain save takes too long, but keep in mind there are still things programs do after receiving a kill signal from the OS (Windows). A kill -9 type signal (like you'd get from an end task from Task Manager) is definitely a terminate-now, but alt-f4 is a slightly less insistent version. Note how when you end task something the first time, windows still gives it a few seconds to respond, but if you end task it again while it's still hanging, at that point you get the option to terminate immediately.

Still, it could very well be too much for the game to handle in a "close-before-exiting-fast" routine. I understand the difference is due to the way the game is designed, I'm just saying, this design can results in some circumstances where corpses are lost.