It did that for my 10240 build install, using only "cliff" for user name.
From my initial research it seems this is by design. *face palm*.
<del>Microsoft is at it again. It also seems you absolutely cannot create a local account anymore without first giving up your email address or a telephone number. I don't remember this hardball approach before.
This monster is relentless. I think they only take steps backwards to let users cool off, then when they think its safe to tread ahead, they start up again.
I know that you can create an account and switch back to a local one. But it's ridiculous that you can not go straight to what you want.</del> How do I get HTML tags to work in here dammit!
Okay, I figured it out. On the screen where it appears to be forcing you to create a new Microsoft account, there is a link at the bottom to bypass doing that. So, essentially buried in the OS and offered as a way to create a local account only after jumping through some hoops.
Also, figured out how to get the /users/(name) that I want. Create a local account only first and you get the actual name you want. Then give it administrator rights, Later on, you can then "link up" your Microsoft account to it again if you want.
I did this and then went back and deleted my 5 letter account.