Simply put. Your "winnings" is the amount of money you have won in total in your career. Your "credits" is what you have left to spend and is made up of your winnings ($1=100 credits) plus anything you've bought using a credit/debit card minus anything you've spent in the pro shop.
Your winnings are purely a for information statistic, your credits is what you have left available to spend. Hope that clears it up. T