Nightblades suprise attack crit bug.

Stannum

Damage from critical attack with Suprise attack skill from invisibility is about 2 times less then from stealth. 252hp vs 522hp

Sharee

Sneak attack and attack from invisibility are two different things. One gets sneak attack bonus, other doesn't. Sneak attack damage is bigger than even a non-sneak critical hit. This video explains it:

http://www.youtube.com/watch?v=Eur7R3791Ww