On many CiviCRM-Linux installations, the Apache PHP process runs as one user (www-data) and the CLI tools (drush, phpunit, etc) run as another user. For files in Civi's data directory (e.g. the log file), this can produce permission conflicts.
Linux file ACLs (setfacl/getfacl) allow sysadmins to address this situation by setting "default" permissions on a per-directory basis. With default ACLs, it doesn't matter which user creates the file – both users end up with the right permission.
...unless someone changes permissions away from the default. This seems to be the case with our log files. Specifically, Log_file appears to change permissions.