It's not plausible to me that, if they were indeed trying to scale the difficulty of the entire adventuring day up in tandem with increasing encounter difficulty, they would recompute and then retain the "six to eight medium/hard" text, but then forget to update the whole chart that comes along with it. Human minds don't work that way--we see large visible charts more readily than we see captions for those charts. It's more plausible IMO that they weren't thinking about adventuring day difficulty at all there--they were just trying to relabel the difficulty of the individual encounters, but not try change their recommendations for the adventuring day.
