Why do you suppose that Jordan does not believe Gatsby when he says that he had attended Oxford College in England?

Like many other people, Jordan doesn't believe all the hub-bub about Gatsby. She seems to thinks all the information bandied about to be false.... one long, endless rumor.

“Now you’re started on the subject,” she answered with a wan smile. “Well, he told me once he was an Oxford man.” A dim background started to take shape behind him, but at her next remark it faded away.

“However, I don’t believe it.”

“Why not?” “I don’t know,” she insisted, “I just don’t think he went there.”