Well it seems like Backstab only do insane damage if you would kill in one hit.
Like some dude has 200 hp, if a backstab would do 201 it does the animation and does a flashy 7000 dmg for no reason (because the max damage is 150% bonus so 7000 is pretty insane) but if it doesn't kill in one hit it just does the '' real '' damage.
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http://en.wikipedia.org/wiki/TORtanic