While the reader can have easily predicted daisy and tom's reactions to his party gatsby cannot why?

Gatsby is "new money", meaning he was not born nor raised in lavish life. Tom and Daisy are "old money", so this party Gatsby had is just a norm for them. The reader is made aware of this. But Gatsby, having not been experienced in this way of life, did not predict their reaction to be understated. He expected a grander reaction.