net calories

I am assuming that you don't do the same exercises every day, and also you don't eat the same foods everyday. The "recommended" calories (assuming you didn't customize them) provided by MFP take into account the minimum you need to eat to stay alive and lose the weight you asked for (per week) when you set up your account. As you exercise you "earn" additional calories based on what you are burning. So you may or may not have calories left based on what you eat and what you did.

Read this:
http://www.myfitnesspal.com/topics/show/10589-for-those-confused-or-questioning-eating-your-exercise-calo