Your username can contain up to 15 characters. Why no more? Because we append your username to your 140 characters on outgoing SMS updates and IM messages. If your name is longer than 15 characters, your message would be too long to send in a single text message.
Your real name can be 20 characters long. Although your username may contain only 15 characters, many real names exceed 15 characters. Since we rarely send real name info via text message (except when using the WHO IS command) we added extra characters for folks (like Konstantin Gredeskoul) with longer names. Real names are also used in follow notification and request emails to help accurately identify folks with user names like cupcake25.
"Don't drop acid, take it pass-fail!" -- Bryan Michael Wendt