I have a PC, that has a User name (names changed for security) ...User name ABCD.
Groing to the Dir C:\Users I can plainly see a directory ABDCZ (again fake username for security)
Other dir under d:\users are the usual that include ABDCZ and the below:
All users
Default user
degault migrated

Going to the control panel, System, I see computer name ABCD-PC and full computer name ABCD-PC.
Going to control panel, user accounts I see ABCD...No other users.

When I login to this PC, from external app, I have to use ABCDZ user name.

How did this happen?