There are two possible Names for any user's account.
1) your username - this is the name you Registered when you joined the forum - it cannot be changed
2) your Displayed Name - this is a Required field in your Profile, so you have to put something there.
Whatever you put there is what will be displayed at the head of your Profile and on each post you generate.

That way you can have all your posts show a name that is NOT your actual username.

This screen snap shows where you enter that Displayed Name.
In my Profile, I kept my Displayed name the same as my username. Less confusing for me.