It's pretty simple maths, the average NRL contract is 3 years, there are 30 + 6 players in a squad, which means ~12 players average off contract every year. Look at any club and that's the long term average, sometimes it's less, sometimes it's more.
It's not mathematical, good teams retain good players for longer now.
Some examples, last year's top 8;
Penrith - 5 confirmed losses 2023 + 13 off contract 2024
Broncos - 4 confirmed losses 2023 + 9 off contract 2024
Melbourne - 2 confirmed losses 2023 + 7 off contract 2024 (the best at retention, hardly surprising)
Warriors - 4 of contract losses 2023 + 12 off contract 2024
Knoughts - 7 confirmed losses 2023 + 9 off contract 2024
Sharks - 2 confirmed losses 2023 + 11 off contract 2024 (interestingly they have not signed 1 player for 2024 as yet)
Chooks - 8 confirmed losses 2023 + 13 off contract 2024 (aren't the Chooks supposed to keep everyone)
Canbra - 5 confirmed losses 2023 + 14 off contract 2024
That's an average of 11 off contract in 2024 for the 8 Top 2023 teams
Always a Bulldog