In The Great Gatsby, why does Nick suspect that Gatsby isn't telling the whole truth about himself?

Gatsby tells Nick the "story of his life" one July morning on their drive into New York. It is replete with details about his wealthy family background, Oxford education, wanderings throughout all the European capitals, and extraordinary heroism during World War I. Nick questions the veracity of Gatsby's tales for several reasons. He knows Jordan Baker believes Gatsby lies about his past. Nick spots some inconsistencies himself, such as Gatsby's citing San Francisco as the place of his Midwestern birth. Gatsby's use of language is stilted and artificial, seeming to be an attempt to speak "upper class." His manner also seems deceptive to Nick:

He looked at me sideways--and I knew why Jordan Baker had believed he was lying. He hurried the phrase "educated at Oxford," or swallowed it or choked on it as though it had bothered him before. And with this doubt his whole statement fell to pieces . . . .

At one point, Nick has to make a conscious effort to keep from laughing at some of Gatsby's wilder assertions. Just as Nick is ready to dismiss Gatsby as a complete fraud, Gatsby produces a war medal and a photograph that seem authentic and serve to support Gatsby's claims. These souvenirs draw Nick into Gatsby's romantic past, at least for the moment. Looking at Gatsby's mementos, Nick gives up his skepticism: "Then it was all true."