Comment by bruceallmighty

It's worth looking at what the stat actually is: http://w3techs.com/technologies

tl;dr: It's 25% of the top 10 million websites according to Alexa over the last 3 months, and only includes top-level domains (So skips counting the millions of WordPress.com subdomains).

If it were all websites, it'd surely include tens of millions of low-traffic WordPress sites - but none of those are used in this calculation.