Determine if outlet is truly 20A
It will be a 20A circuit, according to the breaker panel.

The NEC (National Electrical Code) article 210-21-B-3 has a table in it that allows for 15A duplex outlets on a 20A branch circuit that has one or more duplex outlets on it or more than 1 single outlet. 

That outlet is 15A rated, but the circuit will be 20A rated, and it will have multiple outlets on the same circuit.