From memory, I've seen my own generator DF reading near-100% at idle as well but it's not something I've really studied. When I'm headed out tomorrow, I will take some VCDS logs of what you're looking for.
My dash panel voltmeter is generally stable around the 14 volt level. But, on cold mornings for a short while after startup, it can wander around lower at idle engine speeds. I used to see that a lot running my kids up to the bus stop in the morning over the winter. Do you always see lower voltage than you'd like or only just after startup?
At the voltage levels you're seeing, it's quite likely intervention load management is tampering with high-power consumers in the background. You have to be in pretty bad shape to get a warning in the instrument cluster; ILM is active and working on less-important stuff way before that. Watching ILM state in Central Electronics can give you important clues. If you see voltage changes, it may not be changes in alternator output, it may be consumers silently turning on and off. It'll also be tampering with engine idle speed in an effort to raise alternator output.
Since you have that nice solar sunroof and the mythical jack-kit wrench, I'll just bet you have the heated windshield option too. The current draw for that thing is outrageous, 1000 watts I've read, which is north of 80 amps @ 12V. Is it cold enough where you are to have that come on at startup? I seem to remember reading it comes on for two minutes after startup if the outside temperature calls for it. I don't know if turning Climatronic to "OFF" disables it. But, that's something you can see in VCDS with the new Climatronic labels.