tracking confusion: macros don't add up to calories

I'm wondering what's going on with my tracking, and if I've done something wrong or if there is a glitch in how MFP calculates calories/macros?
I have my goals set to 2000 calories/day, with 40% carbs, 30% fat and 30% protein. At the end of the day, sometimes I've reached my calorie goal exactly, but have gone over on all three macros. How is this possible?