MB ATX connector not the same as ATX power supply.

[sundance]

Any idea why the 24v CPU connector is different on the motherboard (Gigabyte Z370P D3) than the connector on my ATX power supply (Thermaltake SP-500AH2NKW? Do I need to use the 2 pin part that is separate from the other 6 pin connector or is all that is needed is the 6 pin connector? The MB has one sq. and one roundish for the 2 pin connector and the connector from the power supply is 2 roundish ones. Can only send one image, here is the power supply connector.

Thanks