PSG have a lot of potential and I think they don't want to play at full strength every game in Ligue 1, but sometimes the players come out on the pitch in a good mood and they play well, on those days they will score a lot. I think everyone knows what PSG are capable of in their best games, so we can expect so many goals from them sometimes. Their motivation in Ligue 1 is not so high because they have no doubt that they will be able to defend their title again, and they will pay more attention to the Champions League.
PSG can start their non regulars and still finish the season as the title winners. PSG outweighs Ligue 1 toughness, other teams are really no match for them, so why waste strength of the squad knowing there are tougher matches ahead.
We all know that defending Champions League isn't an easy task, if they want to compete at highest level they need to manage players well being, this will help from abstinence of injury as well, most especially on their key players.