aadsas calculates with letter grades whereas many schools use only a number system. so for example, all of the classes in which my school gave me a 3.5, aadsas only calculated a 3.3 because I had to report them as B+'s on the aadsas letter grade system. this page will show you what I mean.
Okay, so that makes sense. Since I go to a school where they only give out letter grades, I didn't even think about the other scenario. I guess there are some advantages/disadvantages to either method.
A UPenn admissions officer came to my college on Monday and ask him about how they evaluated GPAs. He said that they basically, look and weigh the transcripts GPAs higher than the crazy AADSAS GPA calculation procedure. Finally some reasoning to AADSAS madness