In a Drupal installation on CentOS, there are many folders with the following permission:
drwxr-xr-x 8 6226 6226 4096 Jan 7 21:46 cck
The user/group 6226 was not created by me. How come there is such a user/group? Thanks!
CCK is a plugin. Did you perhaps download the tar file and extract it while logged in as root? If so the permissions in the archive would be used on the filesystem when extracted.
To improve security, one normally installs subsystems like Drupal with their own uid/gid. The bug here is that the package failed to create the relevant user and group entries (or, if it was a tarball instead of a distribution package, that you haven't run some post-install script).
6226:6226 permission happens when you are running tar or drush as root. Try running it as a non-root user who is part of the www-data (or whatever your apache user is). Doing this way all files will have ownership as user:www-data