I did a forum search and found: http://forums.carrionfields.com/dc/dcboard.php?az=show_mesg&forum=17&topic_id=16539&mesg_id=16625&page=

Seems to show the funny results I've been getting as well. I don't know what's going on, if there's a weird modifier or only one is getting applied or if they're counteracting somehow, but my ambushes seems to hit as hard or harder without getting death blow than they do with it.

Below is same weapons, same mob, same gear, same level and was consistently showing the same or less damage for DB ambush than regular.

You dive down upon an ugruk guard from your
perch in the ceiling, landing a devastating strike!
You deliver a blow of deadly force!
You step out from your cover.
Your surprise attack *** DEVASTATES *** an ugruk guard!
You are unable to land a crippling strike with your ambush of an ugruk guard.
An ugruk guard is covered with bleeding wounds. (50% - 75%)

You dive down upon an ugruk guard from your perch in the ceiling, landing a devastating strike!
You deliver a blow of deadly force!
You step out from your cover.
Your surprise attack *** DEVASTATES *** an ugruk guard!
You cripple an ugruk guard with a precise strike!
An ugruk guard is gushing blood. (30% - 50%)

You dive down upon an ugruk guard from your perch in the ceiling, landing a devastating strike!
You step out from your cover.
Your surprise attack === OBLITERATES === an ugruk guard!
You cripple an ugruk guard with a precise strike!
An ugruk guard is gushing blood. (30% - 50%)

So, my question is: Is this really working as intended/by design?