All racks do this, it's a combination of several things. First, heat rises so higher tubs tend to be warmer. Second, the tubs above a tub help insulate it. So in most racks, the second from the top tub is the hottest. Third, depending on how the flexwatt is run, some have belly heat and partial side heat (sort of a zig-zag running up each side).

Above all, don't worry! Your snakes are more robust than you might think. If you have a several degree sway between various tubs, try and set all temps in the center of your ideal range so that the highs and lows balance out. For example, I shoot for about 90-92 degrees on the hot spot. I shoot for 80-82 on the cool side. After all was said and done, I have some that are 82-84 on the cool side, some that are 79-82. My hot spots in each tub vary from 93 to 90 degrees.

Sounds like you're doing everything right, so don't sweat it!

JonV