Do you work under the assumption that your aerobic workout boosts your metabolism for hours after you stop working out?
This statement is actually true -- but the calorie burn is probably not nearly as much as you think!
According to coach Eric Harr, author of The Portable Personal Trainer, your metabolism will continue to burn at a slightly higher rate after you finish an aerobic workout. Unfortunately, the amount is not statistically significant. In fact, it allows you to burn only about 20 extra calories for the day. Harr says there is a little bit more of a metabolic boost after strength training, but still he says it is marginal.
Moral of the story: Just because you are working out doesn't mean you can consume all the calories you want.