If you're looking for a serious answer, here's my guess:

Harder matches mean you and your opponent prepare differently for the challenge, better condition, bigger win bonus; more injury prone tactics, arrows clashing from both teams, hard tackle, hard defending, hard attacking. Other suggestions are that an injury factor accumulates over the season, or with repeat match duty (so use subs, cycle players, if you believe that)

Before suspecting the game somehow "knows" when to screw with you [how and why would they care what Q team you play, or what cup round it is, for example] I suspect it is circumstance that is more likely in some matches than others.

Hope that's helpful, it happens to anybody, I've had 3 last 4 matches, only all season.