idk if they’re as concerned about comparing different sites as much as they are keeping the same measuring methods for sites over the historical data period. The COOPs have been measuring only at 6-8am for a long time because they’re just volunteers that have other jobs and responsibilities too. The LCDs have historically had paid observers measuring hourly at the airports for many decades now. So they are keeping the 6hr measuring there to keep the methodology continuous.
I’ve always thought 6-8 hours was a happy middle ground for allowing some compaction, but not letting it sit too long. I’ve always mimicked CON with my observing methods so I’ve stuck to 6hrs when I can. But part of me wishes I started at 8hrs so I could measure, go to bed, sleep 7.5hrs, and then wake up to an on-time measurement rather than setting my alarm for 11z after measuring at 5z. lol