What save editor are you using? I'm not too familiar with using save editors for RPG maker saves, but if I had to guess it could be due to your exp being the same as it was before, so when you go into battle, you get exp, and it updates your level to that of the appropriate level for that exp.
Try only editing the exp that you have. When you go into battle it should have the reverse effect: that is, after the battle your level should increase to the appropriate level for the amount of exp you added.