Zend/Version.php >> 1.11.1 currently. It'll probably be updated in a future release?
Also probably worth mentioning that while XenForo makes use of certain components it isn't strictly written on the framework.
Well, likely just the version used for release (and the current version at the time it was updated). Beta 1 ran 1.10.7. Maybe not strictly required but I'd expect the newer version of the framework to be more stable, of course.
I'm not sure if there's something keeping them from updating to the latest or if it just hasn't been a priority, though I do know one file of the framework was modified from the release version.
In my experience Zend doesn't break too much backward-compatibility wise, so maybe have your app run 1.11.1? Or from the XenForo distribution of the library?