There was a time when there was only the forums.
Therefore when it was necessary to create an account for the other side of the site some didn't bother.
These days, if a person attempts to join the forums the first check we make is to ensure that they have an account on the other side, no account there and the application is refused. The main reason being is that in our experience such accounts are created to spam us.
As to whether it was possible, at one time, to have a different name, yes it was. However there is some connection between the two sides and for that reason we now insist that names match.
As to whether some people have different names on each side of the site, not to our knowledge. There are/were some people who have a different log in name to display name, that was to make their display name match the name that they had on the other side of the site.