How Can Jared Lee Loughner be convicted?

I dont understand this about the law, if Jared Lee Loughner is declared so crazy that right now he cant stand trial.. how then after medication can they hold him accountable for something he did when he was clearly insane off of medication ? If you're not fit to stand trial due to mental illness isn't that a default admission that the person is legally insane? Please explain this apparent contradiction to me.