Weird AFP permissions?

I'm using OS X 10.5.6 and 10.5.7 to connect via AFP to a ReadyNAS Pro running RAIDiator 4.2.5. When I copy files to the NAS, the resulting permissions normally look like this (using OS X's "Get Info" window):

user: adam, permissions: read & write
group: MTE, permissions: read & write
world: everyone, permissions:read & write

(And yes, I have advanced AFP permissions set to grant read & write to group and to world.)

But sometimes they look like this:

user: (unknown), permissions:read & write
group: avahi, permissions: read & write
world: everyone, permissions: read & write

Sometimes these oddball user & group names come and go; sometimes they persist. I can reset them using Frontview, but if I try to deal with them in the Get Info window, the file typically becomes unreadable (until I reset permissions on the entire share via Frontview).

The answer to the unreadability problem is "don't do that", grin, but what about the substitution of (unknown) for my user name, and avahi for MTE? If I decide to restrict read/write ops to the MTE group instead of everyone, then the file becomes unusable because the group changes from MTE to avahi, how do I fix the problem without changing file permissions on every file on the share?

Has anyone else seen this? Any suggestions as to why this is happening, and how to prevent it?