Green tree frog question

[Sanimal]

Hi everyone. I have two green tree frogs in a 12x12x18" terrarium. The lighting I use for them is the exo terra 2.0 UVB 13W fluorescent light. I have it on six hours a day. One of my frogs always sits underneath it and loves it. The other one sometimes sits under it, but most of the time he's sitting on the glass at the very bottom of the enclosure. I'm not sure why one seems to like the light and the other one doesn't seem to like it. Why is this? Most of the time the temperature is 79 degrees Fahrenheit and the humidity is like at 50%. I know it should be at 70%, but there's really nothing I can do (besides misting) because I live in a very hot climate, and keeping it up is always a challenge. So why is one frog always sitting at the bottom?

Thanks for any help!
Sanimal