Tar not preserving Owner/Permissions

I have a file system backup in a tar archive, created with:

cd / && tar -cpz -f mybackup.tgz usr/local

Doing a

tar tvzf mybackup.tgz 

yields:

-rw-rw-r-- otto/otto      450 2010-04-15 22:08 usr/local/alpha-4.5/pkg/conf/PLATFORM/sparc
drwxr-xr-x otto/otto        0 2009-04-01 08:08 usr/local/alpha-4.5/pkg/ui/
drwxr-xr-x otto/otto        0 2009-04-01 08:08 usr/local/alpha-4.5/pkg/ui/src/
drwxr-xr-x otto/otto        0 2009-04-01 08:08 usr/local/alpha-4.5/pkg/ui/src/thyc/
-rw-r--r-- otto/otto     2416 2001-01-13 16:01 usr/local/alpha-4.5/pkg/ui/src/thyc/vid45
-rw-r--r-- otto/otto      569 2001-01-13 16:01 usr/local/alpha-4.5/pkg/ui/src/thyc/vid46

however, extracting like this:

tar xvpz -f mybackup.tgz 

results in the following permissions:

snip...

-rw-rw-r-- 4 otto otto  450 2010-04-15 22:08 usr/local/alpha-4.5/pkg/conf/PLATFORM/sparc
drwx------ 4 root root  4.0K 2011-03-27 23:20 usr/local/alpha-4.5/pkg/ui/
drwxr-xr-x 7 otto otto  4.0K 2009-04-01 08:08 usr/local/alpha-4.5/pkg/ui/src/
drwxr-xr-x 7 otto otto  4.0K 2009-04-01 08:08 usr/local/alpha-4.5/pkg/ui/src/thyc/
-rw-r--r-- 4 otto otto  2416 2001-01-13 16:01 usr/local/alpha-4.5/pkg/ui/src/thyc/vid45
-rw-r--r-- 4 otto otto  569 2001-01-13 16:01 usr/local/alpha-4.5/pkg/ui/src/thyc/vid46

...snip

Notice the second line. For some reason, the directory is now owned by root and the permissions have changed to 700. Its children, who have the same original permissions as their parent, are created with the correct permissions. This is just an excerpt, this is all over the place in the extracted backup.

What could cause this?

Thanks!