Well don't forget, when you look at team's average quality, the figure shown is a figure for the whole team, including subs and reserves.

So it depends on the quality that was actually fielded. I sold a player once and it bumped my teams average quality up by almost 10 points, even though he was from my subs bench, so my actual team that played that day never changed in quality.

But yes, I suppose it depends a lot on tactics/orders etc. It is known for inferior quality teams to beat higher quality teams.