How does Gatsby reply when Nick asks him how he makes his money? Why is this not an "appropriate" reply"?

First Gatsby says that he had inhered the money, later on he gives another answer, saying that it had taken him only three years toearn the money, Nick spots the contradiction and reminds Gatsby what he had said before, basically Gatsby loses his cool and tells Nick to mind his own business. Surely this is not an appropriate answer because it clearly shows that something was not right which turns out to be true later on in the book.