OpenSSH ~/.ssh/config host-specific overrides not working
There is no "global" value in .ssh/config. If you want to have a default value you have to put it in a Host * section:
Host dev1
HostName 10.40.10.41
User my_username_on_dev_machines
Host *
User my_global_username
Note the ordering: The first match wins!
from man 5 ssh_config:
Since the first obtained value for each parameter is used, more host-specific declarations should be given near the beginning of the file, and general defaults at the end.