1. As mmt mentions in another comment, that's the way the U.S. safety codes rate circuit capacity. 2. A 20 A circuit breaker is allowed to trip at 20.000001 A.

1. As mmt mentions in another comment, that's the way the U.S. safety codes rate circuit capacity. 2. A 20 A circuit breaker is allowed to trip at 20.000001 A. Derating the continuous load provides the safety margin for reliable operation, even using inexpensive circuit breakers.