Not every search gets clicked. Some SERPs return enough information that the user does not have to click any results to get their answer. Or, the keyword might be too broad or ambiguous, so the user doesn't find what they were looking for.
When that happens, this metric will appear as a lower percentage.
"Clicked Any Result" will count any option on the SERP: organic results (including videos, shopping, images, etc.) and paid ads.