The relevant part of the laws:
It is an offence if a player:
(...)
- touches the ball with their hand/arm when it has made their body unnaturally bigger. A player is considered to have made their body unnaturally bigger when the position of their hand/arm is not a consequence of, or justifiable by, the player’s body movement for that specific situation.
(...)
In the case of Fulham, they are interpreting the arm to be in a natural position. The argument is that he wasn't trying to make his body bigger as he probably wasn't expecting it to come down off the bar towards him.
In the case of Onana the other week, they have probably said that Onana knew (or hoped) the ball was coming at him and having his arm up like that was unnatural and making his body bigger. The problem is that if you can probably take images from every single game and see players in exactly the same position. Ok, so the ball doesn't hit their arm usually, but it
IS natural for that specific situation.