Game Mechanics: Rogues double-backstabbing at low levels

[phobus]

I've had this happen quite a few times while playing my level 20 rogue this evening:

[Wed Feb 05 23:05:14 2014] You backstab a goblin battlemaster for 185 points of damage.
[Wed Feb 05 23:05:14 2014] You backstab a goblin battlemaster for 185 points of damage.
...
[Wed Feb 05 23:12:34 2014] You backstab a sabertooth tigress for 177 points of damage.
[Wed Feb 05 23:12:34 2014] You backstab a sabertooth tigress for 177 points of damage.
...
[Wed Feb 05 23:22:42 2014] You backstab a goblin battlemaster for 120 points of damage.
[Wed Feb 05 23:22:42 2014] You backstab a goblin battlemaster for 120 points of damage.
...
[Wed Feb 05 23:28:45 2014] You backstab an Iksar marauder for 120 points of damage.
[Wed Feb 05 23:28:45 2014] You backstab an Iksar marauder for 120 points of damage.
...
[Wed Feb 05 23:40:19 2014] You backstab a sarnak conscript for 60 points of damage.
[Wed Feb 05 23:40:19 2014] You backstab a sarnak conscript for 60 points of damage.
...
[Wed Feb 05 23:42:43 2014] You backstab a goblin soothsayer for 120 points of damage.
[Wed Feb 05 23:42:43 2014] You backstab a goblin soothsayer for 120 points of damage.
...
[Wed Feb 05 23:51:38 2014] You backstab a goblin warrior for 111 points of damage.
[Wed Feb 05 23:51:38 2014] You backstab a goblin warrior for 111 points of damage.

You get the point.

While impressive, this shouldn't be possible at all until 55 (according to someone who actually knows something about rogues, i.e. not me).