Been trying to figure out how to explain this:
The 30% is when folks have a URL with three names and tack on the 'www' to it in addition as a forth.
http://mydomain.wordpress.com is fine
http://www.mydomain.wordpress.com runs into the problems that some DNS servers have along with the spiders.
http://wordpress.com and http://www.wordpress.com is fine because they don't use the 4th machine name in there.
The issue is because you actually can use http://www1.mydomain.wordpress.com and http://www2.mydomain.wordpress.com and those would work fine if you had sites there.
It pretains to a bug to the early attempts with load balancing. I'd have to go find the RFC and/ or docs covering it and that's not somethng that can be looked up easily in a search.
edit: I gotta admit that I myself am torn on the dropping of the 'www' bit but only from a SEO position. I had a Google PR of 6 with http://www.tdjc.be When I put in Matt's plugin to remove the 'www' bit, I dropped down to a 4. But the subdomains remained where they were at 4-6. Not a biggie but it is an ego thing.