I don’t think that P&O is stricter, and I think much of it depends on who is on the door on the day.
We went on a Marella the other year and my Dad was refused entry twice on the same night for his attire not meeting standards: he was furious and barely spoke all night, we were trying not to laugh. Then we went on P&O and he purposely wore a full suit, shirt and tie and on the table next to us sat a couple both wearing hoodies, trainers and jeans. After that he stuck to the buffet 😂😂