If you install Windows 10 using a Microsoft account, instead of a local account, it will truncate the username to five letters, and use that as the user directory and a few other things. (you can fix the user directory with this: https://superuser.com/a/955026/310715)
My question, which I can't seem to find an answer to, is why! Why is Microsoft truncating the user name to five characters for linked accounts? To put it another way: why is the truncation taking place and why the (seemingly) arbitrary length?
EDIT: this question was closed as generating primarily opinion based answers. I guess the presumption is that only Microsoft can know the reason why they chose to truncate, and why they chose five characters. But, if they've published their thinking somewhere, or someone has credentialed knowledge, I think there is still an answer out there lurking. Please, if you have information or leads, add comments.