I think its a combination of all of the above
It has something to do with the players themselves -
Like with your scout, if you are looking at a young player, your scout may tell you something like, 'he is developing at a steady pace' or 'a late bloomer, but should maintain level' So this will contribute. The same way in real life Wayne Rooney has gotten better, but Francis Jeffers has not... some players are just better than others.
This is combined with the more you actually play the guy, and it definately helps if you play him in a european game.
So if you get a really good player with a bright future, and dont play him, he may still develop more than if you buy a rubbish player, and play him a lot - it depends on the players capacity to develop, and how much he plays.
Montolivo is a prime example of a great player, he will develop no matter what, but i bought a young player recently, and his stats got worse, even when i played him the whole game.
main 2 things you need to know -
- Some players are just better and will develop more than others regardless
- The more you play someone, the more they can develop, especially if you play them in important games
You can normally tell - if a player is good and young (under 24) and is a regular in your team, he should increase his overall rating by around 3-4 per season. Maybe even more if he is under 20.
I often find if you promote a youth team player, and put him straight into your first team for a season, he can go from 60 up to 66 or something like that.
also, they will start to develop less and less the older they get, so once a player reaches 26, its only really teamwork that will improve. Again, your scout may say something like 'player is very much the finished article'
once they start to get even older, you will see stats like Pace starting to drop off...
capish?