I realize that this doesn't really apply to people who use newspaper, but for people who have substrate with depth to it, is it only the surface substrate you care about or the temperature closer to the bottom?

I've currently got a bit of a temperature issue that I'm trying to sort out due to the season changing, but while measuring, I noticed the probe underneath said 105* while the surface temperature was only 80*(obviously this was on my hot side).

I'm assuming that the surface temperature is the important one? My guess is I need to lower the UTH and raise the bulb to fix this, but I was just curious.