What happened?
If you're seeing less than what your balance is being sent as a payment, it is most likely because you had some saves that were still pending on your payment date.
See: What is a "Pending" save/withdrawal?
While saves are listed as pending into your debt goals, they will still be reflected in your balance but not available to be sent on your current payment date.
Those saves will carry over into your next month's billing cycle since by then they will have posted.