With every level up a team becomes more advanced, players need to stay on top of their game and training in order to develop further and become even more professional. With that, lost stars represent a visual benchmark, while their Total Quality stays the same. This means that your players have lost stars with the promotion but their Total Quality has not decreased.

As you promote to the next level, your player's OVR (Overall Quality) gets adjusted to reflect the new level of play. This is why it may look like your players have gotten weaker with their OVR decreased, but the Total Quality of players has remained intact.

As in real football, a very good player from the second league may just be an average player in the top league at the beginning. This means that stars are linked to your level: The higher your level, the more quality/performance is needed from your players in order to stay competitive.