Bitnodes is quite buggy/unreliable, essentially I wouldn't count on it. I haven't kept exact statistics but my rough guess is that it undercounts nodes 25% to 75%, depending on the exact routing between their prober and your client.

Also, Bitcoin client isn't working really well with IPv6. If the IPv6 address privacy is enabled (practically all Windows machines) it doesn't update its own "temporary" IPv6 address properly. Additionally if you have e.g. a laptop with both wired and wireless port on the same LAN (e.g. docking station to recharge) it doesn't properly pick up between multiple available IPv6 adapters and doesn't track the changes (visible via getnetworkinfo localaddresses).

The general idea was that Bitcoin client is supposed to randomize its outgoing IP address usage to avoid constantly connecting to the same subnets. The code is a little weird, it avoids connecting to the same /16 subnet over IPv4, what it effectively does with IPv6 I did not analyze. But even with regularly changing IPv4 addresses (like vast majority of xDSL deployments, every 24 hours) it doesn't operate well, certainly is much worse than Bittorrent at locating and connecting to peers.