Google +1 WTF

I've not been using Google +1 very much, or for very long, but I just ran into something I haven't seen before: "Cannot +1 this page."


That's using my Google +1 Chrome extension button, which sits next to my address bar so that I don't have to look around for a button actually on whatever page I'm reading and want to bump that way.

As it happens, there's a +1 button on the page in question, and it worked. But I'm wondering why the extension didn't.