I'm not sure this is the case..
When they first randomized the resources, I had a frostwood tree outside my castle that nobody seemed to know about. As a test, I chopped it once a day for 2 weeks and finally it switched.
One kinda has to think about it from the point of programmer's lazyiness:
1) Is location depleted?
2) Yes, then determine if random change occurred.
Otherwise, the server would have to run constant check on every resource grid in the game, leading to waaay lotsa server overhead. Judging from the inventive nature of the dev team, my guess is they'd land on the side of lazy.