I've been paying my Chase cards from this same Chase checking account for even longer than I've been paying the Citi card, and yet Chase doesn't instantly apply my payment to my card balance, while Citi does. This is a difference in when a payment from the same exact checking account gets applied to the balance of a Chase versus Citi credit card, where that same checking account has been used every month to pay both of those credit cards for years. I'm not sure I can explain this any more clearly. But the inescapable conclusion is that this cannot be explained by some general principle of how direct pay from checking accounts to credit card accounts works. Rather, this must reflect a difference in how Chase credit card accounts and Citi credit card accounts interact with a Chase checking account.

The weird thing is that you'd expect that, if anything, it would take longer for a payment from Chase to Citi to clear than a payment from Chase to Chase. But it is the opposite!