My view would be that operating with a higher ACN than PCN will ultimately mean the aerodrome operator needs to perform more maintenance on the runway than otherwise. The benefit to the aerodrome operator of not seeking to increase the PCN is that they maintain more control over the overweight operations on to the runway and can therefore have more control over the way that may affect their maintenance costs. At some point they may see more benefit in having more unrestricted access to the airfield by larger aircraft and may either update their runway data or perhaps make modifications to the runway itself to provide for a higher PCN.
On the subect of the "U" meaning it's confirmed by actual use, actual use may suggest that there is an impact on maintenance costs and therefore ops need to remain restricted. Flight crews wouldn't be aware of this, all they see is that they land and the runway doesn't fall apart, it doesn't mean it's not suffering minor damage though.
This is purely a WAG from me, but it makes some sense.