Breaking the 4000 Barrier

The title comes from a recent post on my main blog Breaking the 3600 Barrier (April 2023), where the '3600 Barrier' refers to a chess rating level. In that post I wrote,

I see that there are four CCRL rating lists. Shown below are the top five engines from three of the lists.

Why didn't I show all four lists? Because the fourth list was based on chess960 games. Here are the top 25 engines from that list.

CCRL 40/2 FRC [C960] - Index

The most striking feature of the list is that the top engine, Stockfish 15.1, is rated over 4000. This is more than 250 points higher than the top rating shown on the '3600 Barrier' chart. In fact the top four engines on the CCRL FRC list are higher then the top engine on the earlier list. I couldn't determine why this is.

[NB: The domain given here (ccrl.chessdom.com) is not the same as the domain in the right sidebar (computerchess.org.uk), although the two sites appear to be identical. The ratings are based on the same database of games used to compute the CCRL 'Opening statistics'.]