It's simple, really.
People use alternate names so they can be someone they're not for a while and not have to answer for it. After all, if they really screw it up, they can simply create a new account using a different email and noboby will ever know, will they?
The ones who post their real names but keep the nicknames have simply grown more comfortable with the surroundings. They're not worried too much about screwing anything up because they've been around long enough to know the basic community regulars, and they know that no matter what they do the regulars understand them because they know them.
I've only ever used one online name other than my real name in my entire life. It's Parminio.
Reason being, I created that username to play Age of Empires with back in 1997. It's actually mispelled. ParminEo is how it's really spelled, but I changed the spelling to have the name.
Parmineo was the lead general of Alexander the Great of Macedonia. He's one of my historical heros, hence the nickname.
Regards,
Tom