Dropped are links from pages that were removed from our index due to various reasons. These include:
Page duplicates: We crawled a "better" page with the same content
Disallowed: The page was disallowed by robots.txt for at least 2 months
Unavailable: The page returned an error 3 times, and the last successful download was at least 1 month old
The domain does not exist anymore
The page has a low URL rating
As such, even though the link itself may still exist on the page, we indicate it as Lost: dropped when we no longer index the page itself.
In general, we can only update a certain amount of pages for every given domain/URL.
Hence, we choose the most qualitative ones and drop the others from our index in order to keep it fresh and current.