Its quite possible for a site to rank number high for a keyword/phrase but yield no traffic.

The question is: Are people searching for these keywords you are targeting?
Probably the keyword is so specific that very few people think of using it or you have not been very careful at optimizing our content despite getting the top ranking.

The keyword is a very specific, non-competitive search term that very few people would ever search on. If no one searches on a page's targeted key phrase no one will find the page, hence no traffic from the top Google ranking. though if traffic comes they are usually targeted visitors to the web page.

To get the most traffic from a top Google ranking, you have to target a search term that lots of people do searches on!