I think this is most indicative of the frameworks used to power the majority of the sites on the web, rather than the language chosen to power a custom application, and that is an important distinction for our audience.
Luckily for the majority sites on the Internet a CMS is exactly what's needed, and very often with more minimalistic custom development (plugins will take care of most of your needs). Thus with WordPress/Magento/Drupal being the heavyweights it's not at all surprising to see the statistics as laid out.
What I would love to see is a similar chart focused on bespoke applications, more like the kinds we would consider building for solutions, startups, and companies here in the HN space. I would guess it would probably hew closer to the stats in the TIOBE Index [0], but again one would want to filter specifically for web application development. Now those are some stats that would be very interesting!