When any team earns promotion, their quality stays the same, however their rating compared to other players on the same level is different to reflect the competition you're up against. Just like when a team from the Segunda Division gets promoted, they will be up against teams like Barcelona and Real Madrid instead of Almeria and Huelva. While the team will have a very good team in Segunda, when they get to La Liga, they may not play as well because of the quality of the teams is higher.
It's just visual because the quality of your players remain the same.