If it's practical, I'd first double-check that they
have posted by tracking down those posts. Possibly those members quit and later rejoined, or their names are familiar to you for another reason. But depending on how busy your guild is, that might be too time-consuming.
Possibly it is a glitch, and the member list has been prone to them before. You can PM a GGN Developer (
wesden_girl) or an online
moderator if its continues.