When the casino taxes your jackpot they calculate 30% of the jackpot amount, not the amount you were ahead at the end of your gambling activity.
You won some money in Vegas, and had to spend some money to win it (your wagers). The following tutorial will show you how. If you've had this experience, there is a way to get the IRS to send some of that money back.
If you're a Canadian that gambles and wins across the border, American casinos are instructed to deduct a 30% tax off jackpots larger than $1,200 before they're paid out to the winner. How to Claim US Gambling Tax Back From the IRS