It was more or less described earlier for the 3770/3770k difference.
Between 2nd and 7th gen K series chips there would usually be a minor increase in maximum boost clock, and a somewhat higher base clock. The rated TDP was generally the same, with the room to do about double that power draw under the right circumstances. K series have unlocked multipliers. With the correct Z series board, they could be overclocked by a few hundred megahertz.
These days the difference is more noticeable in power requirements and the difference in base clocks even more so. The typical non-K chip will have a relatively low base clock, and only be able to maintain boost speeds for a short duration, while a K chip will have unlimited boost duration, very high power requirements, and the unlocked multiplier.
But the key is they are different chips. Some OEMs will limit their boards to their official list of supported chips. This is to prevent people from running chips that will overwhelm the inexpensive VRMs that are typically on OEM motherboards.