Your second sentence is why the first is unreliable: in particular, advertising server software and versions will get you flagged by most security auditors so the PHP sites which don't advertise will often be the largest and/or most interesting — for example, neither Etsy nor Wikipedia advertise that header.
Do you think that's “at least 78% of all sites” or “at least 78% of sites which advertise what they're built with”? The latter case seems more likely and that will distort the numbers massively.
