Even though Omer Mutar is considered Shogeg elsewhere in the Torah here we consider Omer Mutar as Ones (Tosfos DH Ela). This does not mean to say that the action was done against his will, but that even though the action was done voluntarily we do not consider the person who did this action to be wholly responsible for it. This is due to the fact that he did not know that what he was doing is wrong. Rava holds that Omer Mutar is responsible for his actions because he should have learned that the action is forbidden.

Dov Freedman