When making a transaction with a recommended transaction fee confirmation time setting of within 3 blocks, Bitcoin Core displayed a low-medium priority and suggested a nominal fee. Changing only the confirmation time setting to within 2 blocks gave medium priority and allowed a zero-fee transaction.
The expected result of a faster confirmation time setting is a recommended transaction fee greater than or equal to a slower confirmation time setting. In this instance the recommended BTC/kB fee increased, but not the overall fee paid because the priority change allowed for a zero-fee transaction.