It's the same algorithm, but it only considers upvotes by users who registered before (checking the code...) December 13, 2008. That's long ago—only (checking the data...) 1.5% of HN accounts existed back then. Yet the "classic" frontpage is not that different from the main frontpage. This was the main conclusion when pg launched the feature: https://news.ycombinator.com/item?id=607271, and again in 2011: https://news.ycombinator.com/item?id=2073513.

What that tells me is that the forces creating the HN front page don't have much to do with changes in the userbase over time. That's interesting, and I think to most people (me included, and pg probably included) counterintuitive.