I officially don't understand combat

I normally just play and don't pay attention to my combat log, but I was having to rest after some fights that looked like they should be easy. After looking at the log, I realized I really don't get the system - maybe this is normal, but it made no sense to me.

SD scrapper running CJ and weave. Fighting some Rikti and nemesis, and the lieutenant rifle guys seem to be destroying me, but I figure they are just accurate.

Then I kill everything in a group except a Headman Infantry and a Warhulk both bosses at -1 level to me. And I get the following 2 lines that make no sense-

Headman Infantry MISSES! Rikti Rifle power had a 10.00% chance to hit, but rolled a 99.33.
Headman Infantry HITS you! Rikti Rifle power had a 89.31% chance to hit and rolled a 48.82.

How does the chance to hit jump by 80% between 2 consecutive hits with none of my powers dropping and no debuffs on?

Does the Rikti rifle debuff def by 40% (through my grant cover) or is there something else mechanically going on that I don't get?

With just a brief excerpt from that log, I can only guess what happened.

Most likely, since you said there were Nemesis involved, you killed a Nem Lt, the Rikti attacked, the Nem finished his death animation and fired his haxxorz version of Vengeance, and then the heavily-buffed Rikti attacked again.

There are two things going on here. As mentioned above, Nemesis lieutenants use Vengeance when they drop. That provides a 30% hit bonus and a 20% melee/range defense bonus to everything within 80 feet. Judging by the numbers there you were fighting stuff with several Vengeances stacked. If you're relying on defense, Nemesis lieutenants die last. Actually, that's a good rule of thumb even if you're not relying on defense.

That explains the 89.31%.

The 10% roll is because Rikti infantry rifles have a 10% chance to stun, and "chance to" effects display as to-hit rolls. Any time you see a suspiciously round number like 10% or 20% in the combat log, especially paired with a wildly different tohit roll, it's pretty much always going to be a "chance to x" sort of thing.

Vengeance. Thank you both - I was missing that little piece of information. Shows how much I pay attention to logs.

Kinda tough not killing those lts when I'm shield charging in, though.

Anyone able to point me to a pic of what Vengance-head looks like? I don't think I've ever seen anyone use it, but I don't know what it looks like.

I love vengeance on my widows. Gives my widows a warm feeling when someone sacrifices themselves so I can fire it off! What a villian!