Monks have a total of 84% in variable damage buffs. Some can be considered pretty static (GL3, FoF, Twin Snakes, Dragon Kick), but some are obviously on/off (BFB).

In addition, failed flanks will cause a 40/140 = 29% variance on Snap Punch if that's what you're looking at. In sum, it's not surprising to get a very wide range of damage on a Monk, although I'd only expect to see BFB and flanking variances in general. BFB and Flanking result in a damage range of: (1.2*180) / (1*140) - 1 = 54% variance.