It seems something has changed on iOS 7 in regard to the Reader algorithm. I read this topic (and the article from Mathias)
How to enable iOS 5 Safari Reader on my website?
On iOS7, however, viewing it on an iPad does trigger the Reader button, while it doesn't trigger it on an iPhone. On earlier versions of iOS, it was the same between two devices.
Example of where it happens: http://vertigoweb.be/dominique-export/
