It's better to think of the lines on a player card as "reporting events". The event does NOT affect development. Playing time & coaching do affect development.
The lines on a player card, whether they read "Promotion" or "Regular Season" are regular intervals when you get new scouting reports on player progress. "This guy has improved his glove by X". The development occurs whether it's shown on the card or not. Some guys get reports every interval; some do not. But all players who play "improve", however incrementally, whether it's shown or not.