Google would usually rank the pages by date, so page 2 should automatically go UNDER page 1, with a date attached, that is usually how they filter.
Page 2, is technically fresher content than page 1 though. Google won't rank a page higher, just because it's a day older but yes, other factors included, i.e page 2, will help give a stronger message that it's page 2.
Anyway, something else to consider is this: On your board index, you've not taken advantage of some of the SEO improvements in 3.3. For example, you still have last post showing on the board index. This will often link to page 2,3,4 or whatever, using the anchor text "keyword". So you're effectively telling Google, this page is about "keyword", yet you're linking to page 2 or whatever, not page 1.
This was a huge oversight of previous IPB versions, but at least there is a fix in 3.3 for you to take advantage of.
None of these things are happening by chance, they all have a reason, it's just looking closely for the reasons and not jumping to conclusions. If Matt had simply changed page__st__20 to page-2, you'd have seen little effect, though page-2 would be a stronger indicator to google, that it's page 2, than page__st__20