I believe this is because the IRC RFC states that a 'user' string can have a max length of 9 character. Add to that the ~ prefix that is often used when no identd response is received and you end up with a max length of 10 characters.
Personally I don't know of any IRC server which allow you to have a user name of more than 9 chars, so I can't test this behaviour.
If it can't, mIRC should perhaps be able to interpret longer user strings. However, I feel ambivalent about it.
