It's likely to be related to the accuracy/availability of the monitoring sensors. There are a number of reasons that a cat III installation may be 'downgraded' for a period.
We have procedures so that operations are safe - even if we don't know as much as Robin. If the ILS is declared as only able to support cat II approaches there is a good reason - and the aircraft crew should use the relevant procedures rather than second-guessing things about which they do not have all therelevant information. Same principle applies to controllers of course.