Why would Gamepass numbers keep increasing? As the game gets older, fewer people are expected to play it -- especially with newer games joining the service.Copy sold slows down at certain point, while gamepass numbers keeps increasing. Keep in mind that your list is from July.
The game was still available on gamepass until March this year. That is a long time to get more users.
There isn't any evidence as of now that Xbox had more players for MLB 21 than PS, despite offering the game for "free" to ~20 million players.