Mission chance lower if counter minion?

Hi there, just have a quick question.

I was about to do a mission when I noticed this. I'm a rogue btw, champs in question are ilvl 900. I selected Tess and Marin to counter powerful and minions, respectively. When I just put Tess in, the success chance is 150%, when I put in Marin and he counters the minion and reduces the duration (which I want), the success chance goes down to 132 or something.

This is really throwing me off, as it doesn't seem to make any sense. Could it be because Tess has a trait that increases success chance vs minions by 15%? Or is there another mechanic at play? The first possibility sounds counter intuitive.

TLDR - why does success rate go down when selecting a second champion despite max ilvl and countering a threat/minion?

Thanks!