Do enemies always have a minimum chance to hit and crit?

Just wondering how an Advent Shieldbearer with 70 aim being suppressed by a grenadier can crit a soldier behind high cover in one shot. Mathematically it should be 0 right? 70 aim - 50 aim penalty from Suppression - 40 defence from high cover.

Video: https://www.youtube.com/watch?v=DyxoW-S2ttQ