I don't understand Mastery

I still don't understand. All skills seem to end up at 60% mastery; if it goes by the players min and max attack shouldn't they all end up hitting for the same amount of damage?

This is about magicians? Their formula is a bit more complicated, and the mastery ends up not mattering quite as much. 15% mastery means your minimum is about 0.5~0.6 your maximum, and 60% mastery gives you 0.7~0.8 of your maximum depending on your stats. 90% mastery is about 0.8~0.9 of the max. The largest contributor to damage variance is stats.