Money is a big factor. In addition to knowledge, to get the most extreme performance overclocks you need expensive cooling systems, expensive components (e.g. memory that can handle speeds well above the official specs) and enough money to be able to risk frying your hardware secure in the knowledge that you can just buy more. Other very expensive kit can help. For example, a skilled artisan can lap contact surfaces very smooth and flat by hand, but high-end equipment can do it much better. Money comes into it with quantity, too - if you can afford to buy, say, a dozen CPUs of the same model rather than just one, you will probably get a higher CPU overclock because it will vary between individual CPUs.
Also, performance isn't just about the overclock. For example, memory timings. Someone who really understands all the settings will probably be able to get higher performance than someone who doesn't even with identical PCs with identical clock speeds.
Also, performance isn't just about the overclock. For example, memory timings. Someone who really understands all the settings will probably be able to get higher performance than someone who doesn't even with identical PCs with identical clock speeds.