I just got my Corsair HX650 and nearly died when I saw it had only 23 pins instead of 24 (first time builder). A quick search showed me that it was normal and the 5 volt line was removed in 2003. But the motherboard I am going to get (ASRock Extreme 4 Z77) has the full 24 pins.
If this is the case then why does the motherboard have a full set of 24 pins to connect to and the power supply only 23?
How about this then - Pin 20 (formerly −5 V, white wire) is absent in current power supplies; it was optional in ATX and ATX12V ver. 1.2, and deleted as of ver. 1.3. So it's still on all motherboards in case you are using a power supply unit at or below version 1.2. Believe it or not, some people are still using the older power supply units, not to mention you can still buy psu's in the older versions so the motherboard companies leave the connector in place and active.