PHP probably gets a lot of extra hits in places it might not be used a whole lot. For a long time web host providers had it pre-installed by default. Plus wordpress is used a lot and they can detect that from the admin/edit page.
Also someone posted this thing a while back that does some detection like this to find out what tech is powering the website you are on:
I'm not related to them in any way and I don't know how accurate it is but it was interesting.
I bet they are undercounting a bit since the large content sites will keep their WordPress install outside of public view completely and the site is just headless.
The biggest PHP users on the linked list don't even expose that they use PHP, but it is publicly known (though Facebook uses [or used] their own fork of PHP called HHVM).
I have no particular reason to doubt more than half of sites are run by PHP considering the portion of sites that are WordPress. However, I also have no particular reason to trust that 77% is an accurate proportion.