Checking amps with manual-ranging meter: Jack/dial consistency required?
The 20A socket has its own shunt resistor to common terminal. It measures the current by measuring the voltage over the shunt resistor.

If you select the normal socket for measurement source with the dial, the multimeter may not even be measuring the voltage that is caused by current via 20A socket, or if it is, the measurement will be wrong, since it expects to see voltages in a different range from the normal shunt resistance.