First this is a beta build even if a Release Candidate - You would only get the beta builds if you had previously used the beta version ?

I have complained about this on numerous occasions and the explanation was for those who have been on the previous betas they want as many as possible to take part in the beta releases, hence the auto update.

You can either:
Do a clean install, so there is no reference to prior use of the beta stream.
Or set the Program Update to Manual.