http://imgur.com/a/3HbQm When I attacked the dummy from the back I got 615
http://imgur.com/a/w69oc I did it again My damage was even lower
http://imgur.com/a/aKqp4 I was critting for this much from the front and from the back when the passive was on cooldown
Why am i critting more from the front than the back?
Ashen Rider1 votesAre you sure you just didn't crit when you backstabbed, and then did crit when you attacked from the front?
cPXt3JHpf41 votesno i legit stood behind the dummy autoing whenever the passive reset the crits from the passive were weaker than the crits from the front
TryMucinex1 votesEverything seems to be functioning correctly
A critical strike that results from your items (
) will override 
's passive if the critical chance favors that auto-attack.
The small difference in damage you see there (597-615) is likely because you have a mastery that increases your damage when an enemy is below 40% health.
cPXt3JHpf41 votesbut look at the last picture the dummy health is gone and i got 855 the same amount from the front with the passive down
TryMucinex1 votesYeah, functioning fine still. That critical strike resulted from your critical chance, which would use your original damage (250% critical damage), not backstab's damage (180% critical damage).
I mentioned before, in any auto-attack that would be considered a critical strike due to your critical chance, it will do the 250% damage instead of 180% damage no matter where you are hitting your enemy.
A critical strike that results from your items ( Infinity Edge/Essence Reaver ) will override Shaco's passive if the critical chance favors that auto-attack.