@littleadv was very close, but it's a little more idiosyncratic (I have not made a payment yet, and there were no refunds).
As I anticipated, my bank's first rep who answered my email was clueless and failed to even mention
Full Balance Due despite that being the biggest part of my well-written question to the bank.
I demanded escalation, and here is the response from the second rep:
To clarify the 3 payment options you are inquiring about, the full balance due is the starting statement balance. The starting statement balance is any purchases made during the statement period (10/22-11/21) and other purchases and interest that carried over from the previous statement period (9/22-10/21). The statement balance is the starting balance minus payments made during the period, plus purchases and interest over the period. Pay in full is the option to pay the current amount of interest accrued. Which is the statement balance plus interest accrued since the closing date of the statement period.
So, basically, the crux of the problem is their confusing nomenclature for
Full Balance Due. Despite its name leading folks (me included) to believe that it is more significant than the
Statement Balance, it's actually less than that -- it's just the Statement Balance, but without the payments and interest that occurred during the statement period.
Here is a visual aid I created for myself:
(It contains no purchases because I haven't made any on that card in a long time.)
I've never seen other banks use
Full Balance Due, and I am not sure what its purpose is (especially with the name like that) other then sowing confusion, apparently even amongst the lower-level reps of my bank. With my other banks, it's always just these simple four: Minimum Due, Statement Balance, Pay in Full, and Custom.