I don't know the right words to explain it in english so I'll just try to explain basically like this :

If you have 0% chance to crit and you reach 1% crit with crit stat, you'll gain 0.5% dps
If you have 1% chance to crit and you reach 2% crit with the same amount of crit stat, you'll gain ~0.4975% dps : (1,5*2+98)/(1,5*1+99) ~ 1,004975
...
If you have 99% chance to crit and you reach 100% crit with the same amount of crit stat, you'll gain ~0.3344% dps : (1,5*100)/(1,5*99+1) ~ 1,003344

In fact there are similar mechanics on all stat