Go into practice tool buy some times with storm razor, start hitting a dummy without resting in between the 4th shot will deal lets say ~1000 dmg. Now try again but this time before using the 4th shot, rest a bit for the stormrazor to work and and you'll deal like ~600 dmg
That's probably because 
guarantees a crit but makes your next crit deal 160% damage, while Jhins 4th shot guarantees a "normal" crit, but stormrazors crit damage reduction will apply anyway.
I believe an easy fix for that would be that if you would have crit normally while is up it will grant it's movement speed but won't reduce your crit damage.
Spaghetti Code + Lack of testing.
Can confirm. Went into practice tool. Leveled to 18, bought speed boots and Stormrazor. My 4th shot was always 250 on the full health target dummy without the Stormrazor proc. Always 200-202 with the stormrazor proc. I used two dummies, so I can pound the one dummy to get to the 4th shot, and then use the 4th shot on the full HP dummy (to get rid of any of the %hp missing shenanigans)
Well hot damn. Here I was thinking I got that crit amp on top of the normal crit... no wonder it felt weak.
ye it applies the crit % dmg reduction is so annoying
