Couple DB + crit ring mechanics questions

I noticed 2 strange things with the crit ring and my DB

1. Using fatal blow with a crit ring makes the 1st and 5th hit do the same damage, without the ring they are not. Any idea why this is ?
2. When I'm using mirror image, sometimes the auto kill is triggered, but the monster doesn't die. I assume it's because it triggered on a mirror image hit and so it only does a lower % of the total hp on the monster?

not having anything to do with crit ring, but is triple stab mirror image supposed to hit 2 blade and 1 dagger attack?

I thought Triple Stab was 2 dagger 1 blade regardless?