I'm assuming you were taking the standard deduction before you put the gambling in. The standard deduction gives you a "free' deduction of $15k if single or $25k if married. That is a net positive for you for lack of a better term. The net positive is what gives you the refund before the gambling income and loss is entered.
When you enter your gambling wins, say $50,000, and your gambling losses of $50,000, you lose that net gain of the standard deduction because now that additional $15k or $25k standard deduction gets wiped away and gets replaced by the gambling losses of $50k.
$50k of additional income and $50k in itemized deductions is a $0 net benefit.
$0k in additional income and $15k or $25k in standard deduction is a net gain, hence the refund.
You.go from a positive transaction (taking the standard deduction) to a neutral transaction (itemizing dollar for dollar of that income).
