How are backstab damage rolls calculated?

[Ciderpress]

I've noticed on my currently level 58 rogue that backstab does one of three things (excluding outright misses):

- Hits for the absolute minimum, which for me is 85 damage. This happens very often. Even with double backstab, I'll get many double 85's. Never an 85 and an 86, or 87, or 88.

- Hits for something in the 100-350 range

- Hits for current max which is like 450ish (also rare but obviously welcomed).

The weird thing is that it almost never hits for 86, 87, 88- IE values close to the minimum. It's either absolute minimum, repeatedly, or some value much higher than that. This seems strange, or at least statistically unlikely.