Sometimes I see players with the same age as mine (which I have trained forever and played them heavily in league, cup, B3)... and they've got more experience... do they gain more experience just because they play in a bigger league?
let me guess this are older players, 24 or older? Because till that date the players are created with a random Xp, and you could have rookies and players in starting roosters with high XP. Today rookies start with low XP, and i believe the player in starting roosters have different maximum Xp depending on age.
As a german NT coach and now scout i would say the XP gain today is pretty linear, because nearly all players got the same XP in our teams.