They're overclocking the RAM clock speeds but the board itself might not support those speeds, so it's running but not at a 1:1 ratio with the CPU. This can cause some latency issues. Anyway, unless you're a hardcore enthusiast (which given you don't know the answer to your question I take it you're not) doing this is not really practical nor cost effective. Anything above 4133MHz is difficult to configure and quite unstable if unchecked temp wise. Thus why this is for enthusiasts only. If you're just making a gaming PC and want good speeds, 3200 - 3600MHz kits are the best consumer level speeds.