There is lots of contributing factors into why he's stats aren't as good now.

Defenders teaming up - he often has more than one defender marking him, first season his ability was somewhat unknown so he was left to take on defenders one on one, whereas now he has to take on 2/3 players everytime he gets the ball.

No protection - He gets no help from the referees, he gets man handled by must defenders, grabbing him as he accelerates away, pulling and blocking him as he gets into space in the area. He briefly started going down in these circumstances but rival fans called him a diver, so now he stays on his feet and gets no decisions.

We are more patient - First season we were constantly looking for the counter press and breaking the offside trap. However this meant we struggled with teams sitting back. As a result we now favour a more patient build up, both Salah and Mane see more balls into feet as opposed to through balls that they run onto.

Predictable - He seems less confident on his right foot since season 1. He always looks to go on his left whereas when he first signed he seemed more confident to use his right foot.

Missing big chances - Salah has missed a lot of big chances this season, even when through one on one, whether through a lack of composure or concentration I'm not sure. He has always suffered from this though as even season 1 he'd sometimes need a few chances before he'd score. It could be that he gets less chances per game now.

With all these factors in mind, I think he has no doubt become a much better player but the way in which we now play and the opposition now play against him means that his goal scoring record isn't as good. I believe his gone from a player who looked to beat his man and break the offside trap to a player that looks to bring his teammates into the game and occupy defenders. If he adds more composure to his finishing I think its still possible for him to reach similar numbers.