did you EVER allow guest posting? in testing or anyting?
have you deleted any users? if they had posts, they will revert to being from guests…
but even if any of the above, not sure how wp users and sfmembers would get off… unless you had some really old users from a few versions of sp (4.3?) where it was possible for users to register and not get added to sfmembers until their first visit… that bug was fixed long ago, but perhaps some of those users remained? well, no… the count would be the other way in that case…
have you ever removed wp users other than with the wp api? ie, the wp delete users panel… most odd… have not run across this before…
but yes, you should be able to just delete the ’empty’ sfmembers entries… please make a db backup first, just as a safety measure…