The problem is when they already played against your team (1 or both matches) when they were playing seriously, but played the 'tanking' way against the challengers in your league. What if this cost you the 1st position or the 4th position? Do you see where the unfairness is in there?
To some extent, the same applies for teams that declare forfeit when they have to play against you, but played the matches against the other challengers. This is where some users have reported that the goal difference affected their teams' positions.
The above 2 cases are rare though, especially the 2nd case, but they do happen. This has already been reported by some users here in this forum.