Why does Google index blogs faster than other sites?

On February 26, 2009, Google software engineer Matt Cutts collected questions on Google Moderator and answered many of them on video.

Lee Willis from Cumbria, UK asked:

Why does Google crawl/index blogs (specifically sites notified by "WordPress XMLRPC pings") so much faster than a "normal" site submitting a revised Sitemap. What is the impact of that on the overall "quality" of the index?