My initial guess is that is has to do with how the tanks are covered. One is covered by foil and another by cardboard. It isn't so much the material, but perhaps the foil does a better job of sealing so you might try sealing them in the same fashion. It is also one UTH covers more of the 20g tank than the UTH for the 40g. The heat from UTHs has been known to dry out substrate more rapidly, so you might try turning them off temporarily and see how the humidity reacts.

Also, using a CHE will likely reduce humidity as an aside. They do an excellent job of raising ambient temperatures though.

On an alternative note, what are your thoughts on the biodude terra firma? I looked as his products once, but remained skeptical seeing as when they were still called Genesis Exotics, they got some less than spectacular reviews. I'm not sure the exact reason for the name change though.