It takes some time for the credit limit to update, could be a few to several days. Payment crediting and making credit available aren't treated the same I guess. Its not a thing when you make payment and the credit line is made available right away.
See "When Payments Are Due" in the "HOW AND WHEN TO MAKE PAYMENTS" section
"We may delay making credit available on your account in the amount of your payment even though we will credit your payment when we receive it."