UserAgentClass fails to return an OS for 7.5 user agent strings.
We initially added detection specifically for Windows Phone 7.5 (in http://dev.piwik.org/trac/ticket/2808), but later decided to consolidate and drop the OS version (given market share, and general inconsistency; Windows desktop OS versions continue to be detected though). http://dev.piwik.org/trac/ticket/2824