I have a feeling that rooftop solar is being measured as the hole in generation, but not to its production side: we don't directly know the kWh burned onsite and not sold back to the generators and I suspect we may not adequately measure power used in the local network being backed, also except by centrally managed demand reduction.

Not that modelling isn't good. More that I think uncertainty in demand is quite large. It may be for central planning the only thing which matters is planning how much to make centrally, but it's a bit like economics: if people never bank it, and don't declare it, how big the grey or black economy is, can be a bit unclear.