The users that appear to be ignored are users who have been deleted. This is from a long time ago when the forums were on different forum software. After they switched over, for some reason several user accounts were deleted or didn't transfer properly (eg. "Like A Preserved Snapcase on Stage" was too long, and got lost) MarkG is the one you'll come across most often, but even then if you go digging back a couple of years. Not sure what happened there. Probably he set up a new account on the new forum to test it (he's the co-owner and admin) and then transferred the names across, and the software saw a MarkG already there and couldn't overwrite or something.
All his posts 'recently'* are visible.
* I say recently, it has been ages now. Three years+? Someone will know.
As for celebrating... I don't know.