It's based upon someone's decision, but, also upon how common the name/word is and the sound of it. Say you have the username cold(yes, him) it's a common word and isnt that isn't likeable or dislikeable in what the word means, it sounds nice and is one word. Then take say, complexity(yeah,you) not a very common word, kinda hard to get a okayish shortnick (comp sounds stupid), but the word still sounds okay. Personally I like 1worded/names without any numbers in them. Uniqueness makes a better username.