Seat configurations probably. They have a team of people that work on tweaking seating configs to allow more seats. Take away 1/2" per row for 100 rows, and you now have 50" of "new" space. Remove X inches in width per seat and Y inches from the aisle(s), and you suddenly get 1 additional set per row. I don't know what actual numbers are, but that's how they play with it.

I don’t know if it’s a difference between the models or just configuration but I had 2 SWA flights last week. The first was in a MAX with SWA’s newer configuration and seats with very short tray tables. The second flight was on a 738 that had the older leather seats and much deeper tray table. Again unsure if it’s just that the 738 is incompatible with the new seats or they just haven’t updated it yet.