It appears to me as if Google have changed how they are handling supplemental pages. Whereas before all the Supplemental pages would have the same cache date, I am now seeing various dates, and some as recent as 10 days ago.
What I think it might be isThat google realised that by having such an out of date cache, they were losing out on a LOT of good content. A prime example is a site I worked on. The pages were all pretty similar, same meta description, graphically driven and as such would be seen as far too similar to be worthy of listing multiple pages.
We worked on these pages, gave them all unique dynamic descriptions from the titles, rewrote titles, re-wrote content, added outbound links to on topic pages, made the page worthy in short!
Now here is the thing though. Google was actually, by having a non up to date cache in their index, doing their customers a disservice. They were in fact spamming themselves, as they were kinda having a weird quasi cloaking experience whereby they would show their customers one thing in their SERP’s and serving up what was on occasion a totally different page.
It was said inside circles that we could expect to see the supplemental index brought more up to date, but this is I have to say a revelation.
I would be interested to see if anyone else has seen the same thing happen across their sites.