Ah, that's a good point. Each unit's stats are calculated like this: Base Stat + Level Up Stats.
The Base Stat depends on the class and rank of the class (a Knight with class rank 5 will have higher base stats than a Knight of class rank 3, a Fighter with class rank 5 will have certain base stats that are higher (ATK) and certain base stats that are lower (HP, DEF) than a Knight).
The Level Up Stats depend on which class you are when you level up the character - a unit that levels up 10 times as a Squire will have a much lower ATK than a unit that levels up 10 times as a Fighter.
If your Knight is rank 5 and Fighter is rank 3, then there's definitely a base stat difference that will drop the stats.
We'll test it to see if there are any other stat bugs affecting it. Thanks!