You paid it down mid-month and the charge barely moved. Here is the arithmetic that explains why, and the two rules that require your issuer to tell you which method it uses.
Two nearly identical words defined in two different regulations. One ignores compounding by design and the other exists to include it. The definitions settle it.
Rate, balance and time are the three inputs, and the balance is the one nobody explains. Here is the arithmetic, and the rule that puts every input on your own statement.