According to the Renault website you could get a sunroof in a Clio, but it was not an option for either the 172 or the Cup as it was "greyed out" on the configurator page, this was due to extra strengthening in the roof apparently (maybe just an excuse as Renault couldn't be bothered adding sunroofs).
However, there were numerous reports (backed up) that this option existed originally but was later removed. So chances are it was ordered before it was removed as an option, or it was fitted by a previous owner at some point. I'd be inclined to think it would be the former.
This would explain why there really aren't very many with it, but also why this car has one