K versions of the processors have unlocked multipliers, which means they can be overclocked. However, the
K versions also lack some features, such as the virtualization extensions. I myself am confused as to why Intel removed features in their top-of-the-line processor that lower-end models include. The unlocked multipliers will explain the performance difference-- the i7-2600's are 3.4Ghz processors running at 3.4GHz, while the i7-2600K's are 3.4GHZ processors which are often performing in excess of 4.5GHz when they're being benchmarked. The speeds on the page you linked are simply stock speeds, and do not reflect the speed of the processor when the benchmark is being run.