Quite often you can see a picture when the keyword brings two or three times more traffic than its actual search volume. What is causing this? The reason is that sometimes people click on more than one search result when searching for a keyword, hence the higher number of clicks, while the volume is lower.
Sometimes search results don't contain any good information, and you need to click on a few results to get what you need. That's why "clicks/per search" metric is a better indicator of the true traffic potential of a keyword than the search volume.