This site is buggy and undercounts the nodes. I still occasionally use it, but the numbers there are not even statistically reliable, they are skewed downwards.

Part of it is because of simple bugs (e.g. incorrect pruning of inactive nodes) or routing problems on their end.

Part of it is because of some rather strange ways it attempts to deal with Sybil attacks and cannot reliably distinguish them for legitimate address reuse (e.g. CG-NAT or certain forms of IPv6 deployment like DS-Lite).