They are wildly inconsistent. I tested our RO water for giggles before it got tested by the company and it showed 15-20 nitrates. The RO company came back with 2 nitrates.
Try mixing the old #1 with new #2 etc and see what you get.
Why not sand the perimeter of the flat? At most that looks like a 1/16th. Level is not "true".. leveling can be a dead centered bubble but can easily be off an 1/8th over 4+ ft. That's why I love dead on measurements.
Corydoras are community fish so usually not a problem. Pygmy Cory's like to hang out in the middle of the tank as well so different levels of tank activity/swimming location