They think broad matching on their key terms will take care of the tail. The reality is the best broad matches will pick up 500 or a 1000 queries, the tail has hundreds of thousands. * Ad share is % of time an ad from your website shows up for the query (due to broad-match).
This means that the search engine has more chance of success when the the relative frequency of the query matches that of the need. This is due to the matching techniques that they use. All of which matches the data you quote above.