Good catch on the pic. For some reason I thought that was Stand to floor.The lifted area is between the stand and one corner of the tank - if you look at the -picture above. To me there are 2 options - the tank is not 'square', or the stand is not 'square'. Another 'test' - add an inch of water to the tank -and see what happens. (Or more than an inch) - i.e. if its the tank - the water will push the tank down - and be level. If its something else - the water level will not be even front to back - or side to side.
One of those two mating surfaces has to be twisted.
Checking corner to corner with a straight edge (or level If you absolutely have to but those are usually not perfectly flat either) across both dimensions of the stand Will probably show that it was built with a twist into it.