The "Platform" filter in Site Explorer shows the categories of platform types that exist on the linking pages (not the number of links).
The reason why the number does not add up is due to the fact that some categories that exist on the linking page overlap. For example, you can have a Wordpress page with e-commerce functionality.
Also, the sum of those categories is not equal to the amount of backlinks because:
"backlinks" are not "linking pages" (one linking page can have many backlinks);
we only show "popular" types of platforms (Blogs, Ecommerce, Message Boards, Wikis, CMS, Comment Systems, Wordpress), while a lot of pages can be built with lesser-known technologies.