Originally Posted by
Babylonius
Vers and mastery are multiplicative.
The reason is that as you get more mastery, each further point of mastery isn't as useful. At 0 Mastery, you have 10% more damage from Mastery, adding another 1% mastery adds 0.9% total damage. If you're at 20% total mastery, then another 1% mastery adds 0.833% more damage, and so on. The same applies with versatility and critical strike. Because each stat has a different point to % conversion and mastery effects a different proportion of damage than crit/vers, they have different points at which the other stats catch up and pass.