Probably sloppy workmanship. If you put a 2 inch thick piece of polystyrene foam on the stand, beneath the tank, it should help to remove any imperfections in the stand. Most of the weight will be put on the 2 long pieces (front and back) and less weight will be on the left and right sides.
The piece in the middle is more to hold the front and back pieces together and help keep it square.
I can't see all the seems that are welded but if it held 2 tanks before, it should be ok now.
How big is the tank you plan on putting on the stand (length x width x height, and thickness of glass)?
How thick is the metal on the stand?
Is there any rust on it?