[1.2.323] (2) Moar Skill Balancing

XBow damage multiplier = 1.35
Bow damage multiplier = 1.15

1.35 * 170% = 229.5%
1.15 * 200% = 230%

Throw in stuff like xbows having a little higher ATT, less STR required to equip an xbow, greater benefit from with crits, big damage boost from SI/fast weapon, and Snipe, and you'll see that BMs have quite a bit of ground to cover.

MMs also have 10% more mastery.