From what I can gather, this is how they get the League Best performance.
Each team has its own set of best performances in the same categories as the league best performances. The system then looks at each of the 16 teams in a league and gives the best performance based on those result, so the best performance can be from a cup game, a game in another league or a game in this league. I don't think scrimmages and PL games count toward this, but I may be wrong.