I have similar with my strikers, my best quality strikers don't seem to score in league games so I play 2 that do in the league and my better ones in the cups and asso games where they seem to do better, didn't see the logic in this but forgot to change my strikers for 2 league games and lost 1-0 and drew 0-0 against weaker teams with them in. often there seems to be no logic in this game so if you find something that works keep doing it, my last few league campaigns I've had top scorers who were far from the best quality ones in the league, go figure