Not sure of the exact math, but yes, basically there is an algorithm that determines spawn location and amount in a given area (not sure how the area is defined) of EvE based on how much and from where in the area it is missing quantity from a preset baseline amount.
The gist of it is that you are encouraged to move around and seek out the resources, rather than have your preferred location filled every day.
It’s been like this for years. And because of NPC miners, I doubt any belts return to 100% (I understand the only exception are systems without stations. In these systems, NPC miners don’t spawn).
edit: from my experience, a belt will be 1/3 restored after each downtime. So it’ll take 3 days to get back to 100%.