That doesn't seem like a very good way to do crit.

It seems more anecdotal.

It may however be possible that enemies have a crit threshold per level, so for example if you have 500 crit and their crit suppression is 450 crit, then you only get 50 crit worth of points, while if you had 520 you have 70 crit worth of points, an effective 40% crit increase even though you only have a 4% actual crit rating increase.

Therefore the value of crit jumps up past a threshold then falls back down (relatively).