Your comment was like a bucket of cold water thrown in my face, snapping me out of my own smarminess in order to get back to doing something useful. Thanks.
It's a default page provided by web hosting management software. You use Plesk to manage multiple web sites, which may not be related to each other or even use similar backends (for instance, if you're a web host with multiple clients).
The people managing the code and content of the site in question are responsible for customizing their error pages.
