This is probably because the PB and beta 3 were identical builds; the only reason Apple's servers treated them differently is if you had the PB profile installed. Remove that, and you're effectively running the Developer build.
Whether they continue with build number aggregation throughout the beta process remains to be seen, but judging by the fact that Apple have (perhaps unintentionally) made it very easy to jump between DP & PB, I can't see there being much consequence for updating when you're not a registered developer. Developers need the license to sign their apps before they can test them anyway.
Actually, non-devs can test apps now if they download the Xcode 7 beta.