I think it is scientifically proven that acrylic tops do absorb moisture on the exposed side (side facing tank water) and warp/bow as a result (something about expansion from absorption). Not sure what the tank itself does/has, that makes it not warp from the moisture (maybe it's just the way it's put together or the thickness or some kind of coating?).
But I have read of many people who never had warping of their acrylic center brace, but once they added a light fixture (that gets decently warm) directly on top of the tank, the center brace would then start warping (as well as develop crazing in the corners of the open top areas. I, myself, got a used acrylic tank with a warped center brace, I left it to bake in a hot van during the summer months and the warped brace was flat and level again. So I do believe heat has an affect as well.
I swear though I have heard others and myself as well in the past, use acrylic tops with no direct heat on top of them (light fixture) and didn't have warping, yet there was plenty of moisture on the acrylic lid sheets/panels. I guess it just depends on many factors (thickness, amount of moisture/humidity, heat/water and air temp, length of lid and distance/span of the open gap, etc.).