That's however Nostr.band defines it. Think it's something like page rank.
Discussion
Ok but how do we know this number is accurate without knowing the definition nor if that makes sense in terms of retention?
The former is a good question for nostr:npub1xdtducdnjerex88gkg2qk2atsdlqsyxqaag4h05jmcpyspqt30wscmntxy, retention though is definitely meaningless if it includes easily generated keys and profiles.