My reason is a mixture of Phlexor's and SD's.
When people change their avatar and username, I try to find out who they are, and will think of them in their old name. Sometimes people are called by a name I've never seen as their username, and I start using it too, like Hadron, much shorter than Christopher McCandles.
And like SD, calling someone Fatty McBlob, Splatface, Blarch etc, just is not nice to use. And hard to remember too. When I see a pic of you, I don't think of those usernames. Maybe if you had a face that had met a sledgehammer, it could be done, but you don't have a face like that. Fiona seems the most neutral to use for me.