The CPU connector on your motherboard is different to PCI-E power, on a motherboard you have the long 24-pin ATX connector and a smaller connector that's either 4 or 8 pin which is why PSU's come with 4+4 CPU power so you can use it with both types. PCI-E power these days is usually 6+2 pin AFAIK so it can be used with both 6-pin and 8-pin connectors. For example, my GTX 980 Ti requires 2x 8pin so I have 2x 6+2 pin cables plugged into it but my old card required a 2x 6pin so I just didn't clip the +2 pins onto the 6-pin connector