I don't understand the rationale, but currently Lithium serves up slightly different versions of the mobile site depending on the device. It does this by examining the WURFL
The primary difference is the number of tabs. When I last checked (in December last year) Blackberry and IOS have 3, whilst everything else has 2 (this may have changed by now however).
I'm not aware of any other structural differences. However, there are apparently slight differences in the CSS for devices with the following WURFL IDs: blackberry_generic, generic_ms_phone_os7, generic_windows_phone_os_7, netfront_v3, and nokia_gener
There's a useful page with information about the mobile site here: http://lithosphere.lithium.com/t5/documentation/Admin-Guide-Lithium-Mobile-FAQ/ta-p/5885
