It might be becasue the the way zoom is implemented in PM, at least on my version 4.5.4, the Z keyboard shortcut is a toggle. It toggles between two states: *zoomed and default view. You set the *zoom level on the slider in the right pane in preview (look under Zoom section) to anything from 1/4 (25%) to 8 (800%). The zoom level you set on the slider is considered as the "default" zoom in level. So the Z toggles between "no zoom" (usually *fit to window size*,or whole image) and "default" zoom. You can see the behaviour if you open preview and toggle the zoom checkbox or press z while the right pane with zoom settings is visible (press f or r to restore the right and bottom panes in preview mode). On the contrary, the +/- buttons do not change the "current" zoom level, but advance/retreat from the "default" zoomed in level, WHILE toggling the zoom checkbox at the same time! Which I admit is a bit counterintuitive, and we could file a feature request to Kirk to change it to more logical intuitive behaviour.
For example, currently, at least in 4.5.4 (which is my last version, I haven't upgraded to 4.6.x yet unfortunately), when you open preview you see the whole image. You have set the default zoom in level to 1 (100%) on the slider in the right pane. So now clicking the zoom checkbox or pressing Z will toggle between unzoomed state (fit in window) and the 100% zoom. Now say you are in the unzoomed state (checkbox unchecked) and press +. What happens is that the checkbox is now checked, and the default zoom advances one increment to 2 (200%), which is at the same time set as the new default zoom in level. Or you are unzoomed (checkbox unchecked), default level is 100%, and you press -. It gets checked and the default zoom level retreats one less to 50% at the same time, so from unzoomed (fit in window, which might be something like 13% on todays monitors and 10-20megapixel files) you get straight to 50%.
I hope I explained it clearly.
I agree this behaviour is counterintuitive.
In my opinion, the Z toggle should be the master, and always toggle between hard set "default zoom in level" (which 99% people leave at 100% zoom) and fit-in-window. The +/- keys SHOULD NOT change the default zoom level at all, all they SHOULD DO is change the current zoom level one increment, WITHOUT TOUCHING the default zoom in level at all! That way, I can use the Z toggle to always go from zoomed out to default zoomed in (for most people 100% or perhaps for some 200%), while being able to TEMPORARILY zoom in or out using +/- keys without changing the default zoom level.
What about it, Kirk :-) ?