You know how sometimes hunger grows gradually, and you go from being a bit snackish to being moderately hungry to being ravenous in a clear and orderly fashion? And how other times, you're wandering around not-hungry as a clam, and then WHAM! Your insides start digesting themselves?
So, I got home from class early today, but I'd eaten a big lunch and wasn't hungry so I put off making dinner even though it was already half-prepared. And so I poked around doing a little bit of nothing until suddenly I needed to eat so badly that I couldn't even wait for the water to boil, and ended up eating half a box of dry pasta.
Now dinner's about ready, because I went ahead and made it anyways, but I'm not hungry because I have half a box of dry pasta slowly expanding in my stomach acid.
And the moral of the story, kids, is that your stomach is an idiot. Don't let it tell you it's not hungry.