You need a certain ammout of mastery vs a certain amount of haste to get something like what SoulSeeker is getting. I have around the same mastery he has, but my focus is still on haste gearing, and I have a dmg source output like yours Forbearance. In otherwords Mastery gets better as you have more(mastery procs more mastery, and more mastery procs even more, and so on, at 100% mastery you would probably hit only with mastery and disconnect/crash the server =)) ).
@taralej: Haste comes out to more melee attacks, it's just that you also have more haste and agi unlike Forbearance.
@Soulsseeker: No ICD means it procs continously, which it doesn't. :)