New transfers can often come onto your team with poor gameshape. Especially if they were bought as a free agent from an inactive team. Also some higher salary players tend bought off lower tier teams can have poor gameshape since the manager was starting them every game. Looking back at the history of the two transfers you are talking about that would be my guess as to what happened. You bought them with subpar gameshape, then after a few weeks of proper minutes management their gameshape got up to strong/proficient and they started to perform like the rest of your players.