When you quit a singleplayer world, the game will recognize you as "gone" for a little while before the chunks unload. It's also possible that when entering, the chunks load before the game recognizes you as "there", there is no way to tell the difference. This is very annoying when having command blocks check for players online, they will register no players online when you quit the world. To replicate, just hook up a testfor command to a 20 Hz clock, then quit and re-enter the world. The command will momentarily not find you.