SharePoint is no longer a loner standing in the corner but a fully integrated Microsoft 365 suite member. Group Site Members are part of the Microsoft 365 group associated with the site, which includes access to various Microsoft 365 services such as Planner, mailbox, and shared calendar, in addition to the SharePoint site.
On the other hand, the Site Permissions settings only display all users who have been granted access to the SharePoint site, including those who may not be part of the Microsoft 365 group. These users who are only granted access to the SharePoint online site do not participate in Microsoft 365 suite resources.
Hope my answer helps. Feel free to comment here if you have further concerns regarding this issue.
Have a good day:)
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