In reply to Samuel "Otto" Wood.

Since most sites (pretty much all) get at least one visit from Googlebot, BingBot, or any one of thousands of other bots, it's likely that pretty much every site in the universe that is accessible by the public will be hit and will make an update.

Remember, unless you tick the box to keep a blog from being announced to the world, you will always be known by the search bots, and they do tend to show up from time to time.

So yes, any site that is turned on is likely to report. Perhaps not every week, but often enough to skew the results. Perhaps it would be more useful to consider additional items (like say number of posts, or posts in the last 365 days) to consider if a site with the default themes is in fact a valid site to count. After all, a 2 year old site with 2012, and a single "welcome to wordpress" post isn't exactly an active install, even if a bot does hit the page every week.