AI gem match choices
So... fighting in Training Grounds using my level 1 Vraska, Relic Seeker versus a level 7 Angrath. Opponent's turn. Angrath's mana bonus for blue at level 7 is 0, green is -3. There's at least 1 blue match on the board and some other colours. The AI chooses to match green for 0 mana instead of blue for 3?....
What's going on here? As Vraska, my best colours are green and black. Is the AI placing a higher value on an uncertain chance at denying me my colours? That shouldn't take precedence over getting his own cards cast. Or, is it the fact that I had an activated gem (Honored Hierarch) on one of those greens? Because I've never seen the AI target my activated gems over getting their own mana before. Can anyone tell me why this might have happened?