Mctayto

Metallica have deliberately thrown games in clan challenges for years where the outcome is already in their favour.
This wins them the challenge plus reduces their individual member ratings for future clan match ups.
They reduce their rating by say 20 points instead of increasing by 20 points thus one thrown game alone distorts their true rating by about 40 rating points.
eg If their player is rated 1710 and throws the game they are now rated at say 1690 instead of 1730 by winning. This is compounded by the volume of games they throw.

Metallica win net points of approximately 80% of the gross amount (this would be even higher if they did not deliberately reduce members ratings)
If a clan is playing fair then they will have a maximum win ratio net to gross of 50%
The fairest clans of all tend to have far lower percentage than this as they are also being duped by the likes of Metallica.

Be Warned, avoid Metallica challenges like the plague.