It means that people at slashdot are more likely to read comments on topics, as opposed to accidentally being driven there by clicking on the description instead of the title link.

(the entire description text of a story links to the comments page, its very easy to click it accidentally. Additionally, the comments are sorted, unthreaded, by number of diggs by default; unless you have an account and are logged in the landing page is completely incoherent. Compared to slashdots moderated default comment pages).

You are conflating impressions with clicks.

The only thing that would make such a large difference would make sense is the number of people using adblock is much higher for Digg users than Slashdot users, which doesn't strike me as very probable.