I think I got it, Qui Gon asked Shmi who was Anakins father. Qui-Gon already knew about the high midiclorians in anakin. Therefore, knew that there was a human person who must have impregnated her. Qui gon is not stupid. Shmi couldn't explain what happened, meaning in a sense that it was a one night stand sorta thing. A sith lord tricked her to believe he was a good guy than he left her.