Well, 100% crit means always crit. For example, with 2 magess dresses, my archmagi do just that - crit on every hit.
However, the percentages of evasion are based on 2 things: what happened in the previous attack and what will happen on this one. If on the previous attack the unit has evaded (even a retaliation), the 20-30% chance is active for this attack. Sadly, in KBAP, this sometimes means a bit too often. Still, it is also a bit too often for you as well if you use those units.
So we can't complain I guess
Although it is both irritating and frustrating when the AI decides to give you some really hard time evading almost every second attack.