It could be the tank itself, but chances are it is the stand. Have u put a straight edge like a 6' level across the hole front of the stand? Something long enough to do from corner to corner. Did u check the stand before the tank was on it? It won't work as good as a long straight edge but u can use a string to hold from corner to corner & it atleast give u a idea without moving the aquarium.
As far as the measurements being different, it is more likely for the top trim to be off rather then the bottom. If the tank has a warranty I would see what they recommend so u don't end up voiding your warranty. I would be certain before u contact them that it is the tank & not the stand that is the problem