If rounding is the culprit and at 10 APoC we're getting 2 AP per crit, that could still explain why 10 APoC works. That would change my expected AP regen from 51.5 to 82.5, or 92.5 including the base regen rate.
Then, the difference between 10 and 20 APoC is pretty neglegible because you're already gaining more AP than you're spending.
Another way to test would be to compare say 8 APoC to 9 APoC. If the rounding is true there should be a noticable difference since 8 APoC would result in around half the APoC regen as 9 APoC. That's assuming you're just casting WW and nothing else to proc APoC.