The picture of the formation is based on the formation he ended last Season with. But once the Season changed some players became out of contract this Season and so do not show because they are inelligible (a player can not play or train unless he's signed).

Therefore the system will fill the formation with subs. So the formation picture during the match will be full but the squad listing will remain with gaps until a manager logs in and changes it.

Additionally, if in the new Season the team does not have 14 signed players the system will add old players to make up the numbers - and they will retire at the end of the season so can not be sold, only sacked.