I've noticed this for a long time now, but the dialogue of initially asking Garth to train you and the dialogue of asking him again are switched.
So currently the first time you ask him to train, the dialogue acts like you've already trained with him before. Meanwhile, if you ask him to train you again, the dialogue acts like you've never trained with him before.
So currently the first time you ask him to train, the dialogue acts like you've already trained with him before. Meanwhile, if you ask him to train you again, the dialogue acts like you've never trained with him before.