is the lumia on the same carrier?
I suppose its possible that a carrier may use different settings for different devices according to what the hardware supports.
It is strange, have you googled about web problems in general for that device to see if others have any weird web or network problems?
you get the same result no matter if it uses wifi or cellular network and using different wifi hotspots?
if it is a device cache issue, I would think making a copy of the skin with a different name and changing to that copy would get around the previous skin being cached