I spotted one of the more insightful comments on this in a recent post in an ongoing WebmasterWorld thread. The post was from user Nutterum in message number 4774005, where he said:
Panda does roll, make no mistake. I`ve seen several site hit by the algo in October alone. I'ts just not in the same way as previous updates. I think Panda is moving at the pace of the recrawl rate, which is understandable.
See that, "I think Panda is moving at the pace of the recrawl rate, which is understandable."
This would make a huge amount of sense. Google crawls the page after 4.2 was released and then, only then, does the Panda get unleashed (in a good or bad way) on that specific page. Which would make a ton of sense more on how this slow roll out is happening. Maybe it also explains why, why the technical reason Google is doing this. Maybe it is more efficient for Google to roll it out based on their crawl behavior?
Google has never said anything about a Panda rollout based on crawl rate, but it would make a lot of sense.
Forum discussion at WebmasterWorld.