This can be a little confusing, but APR (annual percentage rate) is not the accurate annual interest rate. APR is intended to make it easier to compare lenders and loan options, by providing a standardized cost of borrowing.
But while most credit cards are quoted in terms of nominal APR compounded monthly or daily, the more direct reference for the one-year rate of interest is EAR (effective annual interest rate).
The general conversion factor for APR to EAR is EAR = (1+APR/n)^n)-1, where n represents the number of compounding periods of the APR per EAR period (typically 12 months). The result is a slightly higher interest rate, since a 19.99% APR equates to a 21.93% EAR.
While these differences may seem marginal, because of the compounding nature of credit card loans, nominal disparities can have drastic results over a lengthy term.