MrMalty is more of a guesser than a calculator. It's a good guesser that I use all the time with good results, but it's really just a guess. Without actually counting your yeast and testing viability, you have no idea how much yeast you're actually pitching.
The good news is it's basically impossible to overpitch, on a homebrew scale, unless you really try to overpitch. I'm talking like 5 smack packs, and even then it'd probably still make good beer.