PSUs have a few double wires on the 24 pin connector to detect voltage droop over the length of the cable and adjust power delivery to ensure the motherboard is getting proper and accurate juice. This is why a lot of modular PSUs have 2 connectors with more pins at the PSU end feeding into the 24 pin that goes ino the motherboard.
Corsair's SF series use 18 pin and a 10 pin connectors which merge into the 24 pin. 27 wires into 23 pins (the -5V rail isn't used, so the wire is removed) means 4 wires are doubled up, and those are the thinner guage sense wires.
For best detection, sense wires are doubled at the 24 pin side so the PSU can see exactly how much juice is flowing into the motherboard's pins, but bear in mind for standard PSUs the 24 pin can be 50cm long so voltage droop because of the length and resistance of the wire can be a thing. In small builds with SFX PSUs, the distances are much shorter so droop because much less of an issue, and in my case effectively a non-issue. But my SF600 requires those sense wires to be present, so I can't just chop them off. I did the join pretty far up the wire so I can hide them.
In fact, have a look at this in my build log:
Waste not want not Since the actual shape isn't changing I thought I'd get the LEDs sorted anyway and give it a test. Cool white 3528 LED strips on a 5mm thick PCB wrapped around an inner frame so the light shines outwards Just as an experiment, I covered the underside of the light panel...
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I've not updated it in a long time because I've been too sick and too poor to do anything, but the 24 pin has actually been finished for a while now (I didn't flip the PSU in the end, I managed to do some nice layering with a 3D printed cable bulkhead tucked behind a solid plate to hide the entire thing).
