The cooldown is being calculated with the base CD of Flash being 255 secs when it is actually 300 secs.
So instead of it being 300 x 0.6 = 180 it becomes 255 x 0.6 = 191
It isn't a huge deal but you do lose eleven seconds.
15%(insight)+15%(Distortion)+10%(BoL)
Probably intended. That just means it's stacking multiplicatively instead of additively.
EDIT: After calculating it myself, the ionian boots are stacking with the mastery additively but the distortion enchantment stacks multiplicatively. -15% of 300 sec cd Flash from mastery = 255 seconds, 255 seconds -10% of 300 sec cd flash from ionian boots = 225 sec, but the distortion enchantment is -15% of the new flash cd, making it have a 191 sec cd. Probably intended, but it's possible that they're all supposed to stack multiplicatively or additively for consistency, in which case either ionian boots or the distortion enchant is bugged.
