Group permissions are broken?

[Yuri]

User john is a member of both webcamd and vboxusers:
# grep john /etc/group
webcamd:*:145:john
vboxusers:*:920:john

When the file /tmp/my-test is owned by webcamd, user john can touch it ok:
$ ls -l /tmp/my-test ; touch  /tmp/my-test
-rw-rw----  1 vboxusers  vboxusers  0 Aug 15 12:54 /tmp/my-test

But when /tmp/my-test is owned by webcamd, user john gets an error:
$ ls -l /tmp/my-test ; touch  /tmp/my-test
-rw-rw----  1 webcamd  webcamd  0 Aug 15 13:02 /tmp/my-test
touch: /tmp/my-test: Permission denied

Why does this error occur? Two groups seem identical. Just different 
group ids.

Filesystem is UFS: /dev/ad10s1a on / (ufs, NFS exported, local)

Yuri