With an offensive kill rate of 35% and Straight Round, an attack with 90 troops kills 31 troops when it should kill 32 troops.

90*0.35 = 31.5 which should be rounded up to 32, but is instead rounded down to 31.

The rounding up works on attacks with 10, 30, 50, 70 troops but glitches with 90.
It glitches on attacks with 170, 330, 350 troops too.

The 35% offensive killrate is used a lot (in games with "Warriors" in the name), so it's not a harmless bug.

Do you have a game link?

https://www.warlight.net/MultiPlayer?GameID=12475860

Use Menu>Analyse Attack to see it. I also tested it with an actual attack in single-player.

That does seem to be a bug.

Even Google Chrome has the same issue:

suggests a way to fix it.

^ same in Firefox.

Probably this is not a bug. It looks to me that this has something to do with the way computers deal with numbers. First they work with binary code. Second, they multiply and divide in complete different manner than we are used to do. Imo this explains abbreviations you describe.
I am not mathematically (or rather binary ;) ) skilled enough to give the exact explanation, but I found article which can be helpful to understand what is the issue:
http://mathforum.org/library/drmath/view/54338.html

Edited 12/16/2016 01:57:44