My logic here was that the snake will be spending 99% of its time on the floor of the aquarium so you want to make sure temperatures are correct there. Sorry I wan't clear with this.
When talking about temperature control and reptiles, you're mostly only concerned with where the reptile spends the majority of its time. For ball pythons, that means on the floor. If it's green tree pythons, then it's on one side of the aerial perch or the other. Make more sense?