There are a few reasons that there may be a difference between your Account Balance and the amount you are allowed to withdraw. Most often the difference will be due to a bonus that you have been awarded. Bonuses must be played on the site and cannot be withdrawn.
Another reason your Withdrawable Balance may be different is if you have recently made a deposit via ACH bank transfer. You must wait for your ACH deposit to clear your bank account before you can withdraw it on DerbyJackpot. Generally that take about 3 business days.
Lastly, If you have just made your first deposit on the site you are required to wager at least the amount of your deposit plus the bonus you received before you can make a withdrawal.