Howdy folks. First of all, just to confirm the posting that steelman quoted, the notification of updates is something we're looking to improve.
Also, by way of explanation -- as we've written before, we are delivering content in a form that no other online retailer is attempting, and as a result we have unique processes and requirements to get that content onto the Movie Store.
Depending upon the title, and depending upon the studio, sometimes we are not able to get certain features (e.g. a Dolby Atmos soundtrack) in time to include them when we launch a title. This is most common when titles are released for electronic sales "early" compared to sales on disc.
We have chosen to go ahead and release these titles as early as we can, with soundtracks that are still excellent, lossless multichannel soundtracks, because we know many people will want to see them as soon as possible. (Plus, of course, there are plenty of people who don't yet have systems that can play Dolby Atmos anyway.)
In keeping with our aim to deliver the best quality available, we do update titles to add Dolby Atmos when we're able to get the necessary source materials.
Clearly, we'd love to be able to deliver these movies out of the gate with the Dolby Atmos soundtrack, and I hope that will come to pass in time, but for now this is how things are.
I hope this explains a bit!