I'm pretty sure it's not a medical condition, as a lot of the foods I enjoy are naturally low in calories (such as fruit-I eat like 2 apples, an orange, and some blueberries on my cereal, as well as broccoli, chicken, and yogurt). Honestly, I'm pretty sure it's because I tend to over-estimate the calories in food (90=100 to me).
I've pointed that sort of thing out to her-she constantly tells me that she better not hear a toilet flush while the shower's running (god forbid I get the urge to pee while the water's heating).