Registration is not editing. To one of our admins, who set this important field up, they assumed that clicking show on registration would show it on registration and then the user would not be able to edit it after that.
The way I read it now I can understand their thought process.
User registers, selects custom field that is shown on Registration. User never is given option to edit it because User Editable is unchecked. If User Editable is checked and User Editable Once is checked, the user is given one chance to edit it after registration.
So now we have over 100 new members who have a Required field, shown on Registration, not editable that are sending in support emails asking why they can't post on some of our most popular forums since that field is a usergroup promotion requirement.
It isn't a deal breaker or anything but I can see how my admin made the mistake.