If the match prep sessions are a temporary boost to player attributes, then maybe it is capped? E.g. with a Def. Shape session, you can boost the Positionning attribute of all players to max 15? Something like
boosted_attribute = min(original_attribute + boost, 15)
It would also makes sense to have diminishing returns as player attributes are high.
That would explain the results you obtained. Would be a shame if match preps sessions are useless - having to choose between short-term success and long-term development is great for gameplay