That's because you are thinking backwards.This doesn't make much sense to me game-wise. To balance thing it should be exactly the reverse. Areas with less encounters should have fewer rests, not more and vice versa. You have created a system that intentionally exacerbates imbalance. Which would be fine, except it is weird as you seem to be super concerned about balance...
The idea is to match how easy it is to rest somewhere.
It's easy to rest in the city. The inn, temple, or house of healing is just feet away from the fights. After any fight the party can go to the luxury inn or the temple. So the mages have all their spells and the Fighter can Action Surge 4 times in that one fight. You forgot that the Fighter can burn 2 HD to get Action Surge.
So the Level 6 party, in the one fight the wizard can drop 3 fireballs whereas the fighter can use FOUR Action surges for EIGHT actions in 1 fight.