I have the same issue.
Upon import I build standard previews and also smart previews.
When going thru the photos I decided to build 1:1 previews as did a lot of zooming in.
Now I am having exactly the same problem. 2 pictures with exactly the same resolution both raw both shot within 1 sec zoom in differently in 1:1. One zoomes fine the other one barely zooms in at all just a tiny bit.
I followed some advice and discarded 1:1 and also smart previews. Then I created firstly the smart previews again then after that was finished I also created 1:1 previews. In some cases that did the trick in others it did not.
2 things I noticed though:
1. When I take one of those incorrectly zooming photos, open it in development mode and then press z to zoom in it does work fine. When I switch back to library and zoom in on that photo again it works fine. So it seems it is a library problem and zooming in development cures that one pic.
2. When I discard the 1:1 previews and I open the pic full screen in library and then press z it does not try to create a new preview as it should. Usually if an image does not have a 1:1 preview and I zoom in it should create it on the fly but in this case it does not. It just zooms in that tiny bit as it did before, no spinning wheel no nothing.
So the only thing that does seem to fix it is opening in development and zooming in but that is a huge pain in the a... as I cannot do this en mass.
Maybe just to mention: I am using CC 2015.7 and I have activated the option to use smart previews for image editing (instead of originals).
I believe, and see this myself that this is correct behaviour for the smart preview for develop option?
The smart preview is dimension is smaller and will be displayed as such, and not the full size. If I uncheck the option it displays the full resolution photo in the develop module.