I'm not sure it works as explained, but it might. And I can see positive points in that system.
If older players would have skilldrops at a fixed rate, they would indeed decrease constantly and visibly.
When there is a chance the drop is prevented, aged players have a higher chance of staying attractive for teams. Players carreers would actually be lengthened by that, and it is also more interesting for managers to keep older players, and even buy them.
As a player can drop 2 visible levels in a skill, yet doesn't drop again the week after, it indeed proves it can not be a fixed drop rate.
Why a more complicated system? to make the game more attracktive I suppose, and as far as I'm concerned, it's working.
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