I only have one group on my Discourse forum. From the group list, it shows 16 members, however the actual number of members is 10, as correctly shown when going to the detail view of the group. Short screencast:
AI-generated summary
A bug in Discourse group administration has been reported, where the number of members shown is inconsistent. steko initially reported the issue, showing a discrepancy between the group list and the actual number of members. dax confirmed the bug, noting it occurs in groups with deleted members. tobiaseigen shared a similar experience, with inaccurate numbers in the groups list that become accurate after clicking on a group’s settings. The issue is attributed to a cached count getting out of sync. sam and zogstrip have both claimed to have or will add a fix for the bug, with zogstrip providing a GitHub commit link for the fix.
I confirm. The same bug on my forum in all groups with deleted members.
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Yes, that’s exactly my situation, some members were removed from the group.
Off-topic
By the way, visiting your forum I see that you lost the translation of Termini di Servizio…(benvenuto nel gruppo delle traduzioni perse 
)
I have had something like this on my forum for many months, actually.. maybe since the beginning. Not sure if it has to do with members being removed from groups, but when I see the groups list the numbers are wildly inaccurate. When I click to look at one group’s settings the number changes in the list and is then accurate. I use the API to manage group membership.
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There is a definite bug here, cached count seems to be getting out of sync sometimes will add a fix.
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I’m still hoping! 
Pretty sure I fixed it earlier today 
https://github.com/discourse/discourse/commit/79639e2dec11d2ae056e30b2bfd32ecfed6fec25
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