You're absolutely right that several of the rules for what constitutes an assist are pretty arbitrary, which is why there are different definitions for an assist in different places. The Fantasy Premier League is a bit more generous than the IRL Premier League, which is odd, since they're both on the same website (Mendy has 4 in Fantasy, and Sane has 1 for the own goal - I believe an assist is given for winning a penalty as well). I think the rules in the World Cup are different from the Premier League, and in MLS there can be up to 2 assists given for each goal, which allows credit to be given for Silva-type passes that split open the defence, even though the player he passes to doesn't score the goal (of course there will be a knee-jerk reaction amongst many on here against anything they do differently in MLS, but I think it's a much fairer system).