Differences of tar and cpio related to preserving attributes

Hello,
It seems that the only way to guarantee that tar will preserve attributes is when it's run as root.
Performing as a regular user, as below, may not preserve attributes as expected:

$ tar cpv@f backup.tar somedir

By contrast, cpio seems to preserve attributes even when not run as root.

$ find somedir | cpio -oP@ > backup.cpio

Carlos.