“Not graduating” means just that—not graduating. “Withdrawal,” however, is a technical term and an official action that is reflected on one’s transcripts. It is the official act of leaving a school (temporarily or permanently). So withdrawal leads to not graduating, but is technically not the same thing (though they are close enough that the difference is only of formal importance).
Note that one can also withdraw from a class, which is always permanent. Again, this is an official act and is separate from dropping a class or no longer attending.