Hello,
Unfortunately we are not privy to the exact meaning behind game terminologies nor the math governing these actions. Increases to critical rate certainly increase the likelihood of a critical hit, but these increases are not necessarily additive, and there may be a hidden cap of which we are unaware. I can say that, it looks like the unit is working. An 80% critical chance is quite impressive and very good, and certainly would not happen without that unit. However, as we are not aware of the exact interactions between stats or how buffs interface with them, we can't really say how a unit "should" be performing in situations like this.