If you have a striker from day 1, and only train attacking skills, say, then over a few seasons the physical and defensive skills average % will drop.

Based on the above, the attacking skills will go up, but the average of all 3 (Att/Def/Phy) will be lower and make the player a 3* in your case. Despite that, he may have the attacking attributes of a 5* player - which is why the player is still effective.

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