Here is what I found setting up DNS clustering.

Lets say I have server G.domain.com and A.domain.com
I went into WHM on G and set up clustering. I copied the access key of A into the interface and voila, the clustering is set up. I choose "synchronise changes" in the interface and everything seems ok.

I go to A and choose clustering as well but do not copy any access key in there or do anything else.
G shows up as "standalone".

So far everything seems ok, but just wait:

Now I do a change for a zone on G and it is replicated to A - just as it should be.

The just to test I go to A and do a change for a zone and guess what?

The change is replicated to G!!!!!
This is wrong and should not happen since I chose "standalone" and never even copied an access key of G into the interface on A!!!
This is a major security bug and should not happen.

If server A trusts server G then that does not mean G automatically trusts A!!!!!!!

And it seems the setting of synchronise changes or standalone have no impact whatsoever!

I highly recommend for Nick to fix this asap!

So beware doing DNS changes on a standalone server and wonder why they replicate if they should not.