You have to be careful with the terminology. In your example, the adjusted ampacity of the conductor was 21 amps. The footnote at 310.15(B)(16) refers you back to 240.4 which limits the rating of the overcurrent device, hence the branch circuit, to 20 amps, however,
240.4(D) also notes "unless permitted in 240.4(E)or (G)" so the 20 amp limitation does not apply in those case (motor applications for an example).
Bottom line, it depends on what you are trying to determine. The amacity of a conductor and the size of the branch circuit do not correspond. This is a favorite "trick" in test taking.
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