Do not underestimate the factor of "fitting in the team" both in terms if strategy, communication and attitude. Various formations and tactics optimally require specific types of players that fit their function and it's certaintly possible to get more limited quality out of a player for not playing a style that uses all his attribute's. I think wingbacks and wingers are a good example of this, inside forwards for example are very often required to be 2 footed and will often play with backs going high up the pitch whereas with normal wingers the backs might stay back more. You can have a great defensive back underperform by asking too much of him going forward, and a great all round back underperform by keeping him back too much.
Afcourse optimally teams take perfect players for their strategy's. But these strategy's often chance, if only say because a new manager wants to take a different approach, which will often result in some players needing to play a role that is not perfectly suited for them.
Character, culture and languaghe can play a role too. You might not want too take in too much of a joker of a player in a serious and proffesional player group. You might consider to not take a great player in simply because you fear it will lack in terms of communication, etc.