If some, but not all, of the LOGNAME, USER or USERNAME environment

If some, but not all, of the LOGNAME, USER or USERNAME environment
variables have been preserved from the invoking user's environment,
sudo will now use the preserved value to set the remaining variables
instead of using the runas user.  This ensures that if, for example,
only LOGNAME is present in the env_keep list, that sudo will not
set USER and USERNAME to the runas user.