Why does Nick find it so hard not to laugh at Gatsby's claim about how he lived immediately after the war?

Nick finds it hard not to laugh because Gatsby's story is completely unbelievable and rehearsed.

With an effort I managed to restrain my incredulous laughter. The very phrases were worn so threadbare that they evoked no image except that of a turbaned ‘character’ leaking sawdust at every pore as he pursued a tiger through the Bois de Boulogne.