The options for populating that user block are in ACP - Appearance - Style Properties - Messages but there's no option for member number, just registration date, so maybe a template modification or something in a custom style?
I noticed that the IDs of the members on your forum are strange, I don't really know where it could come from, they are however correct because the URLs of your members' profiles contain these same IDs and the profiles are accessible normally. You should open a discussion about it, I don't think it can come from a theme though.