For me it's just the opposite. I get my notification volume set to max, only to find it down at one quarter or half later on (often while the phone has been resting on a table after I turned the volume up).
It's not just notification, other sounds change as well. I used to think that the function of volume keys changes depending on which application I'm currently in (so it affects dofferent volume setting). But that doesn't explain why the volume changes with the phone at rest.
It must be an app that I have installed that is messing with this. The question is which one...