Thinking laterally, maybe the issue is not one of measurement accuracy. It could be the route the aircraft was flying (well, at least attempting to) that made the controller think "this guy is following the arrow". You haven't said if the pilot suggested the controller might have been mistaken, or if he was skimming the zone; maybe he just flew straight into it (literally following the arrow) without paying any attention to the airspace warnings on his GPS. It could have been not even an aeronautical GPS, perhaps a Tom-Tom or something. Less likely, it could also have had an outdated and erroneous database.
Just food for thought. Btw, there was a discussion not long ago (with interesting answers) about radar precision. Hopefully someone with better luck will find it and post a link.