Motion Blur as PostProcess necessary?

I am noticing that I get motion blur WITHOUT enabling it in my PostProcessVolume.

So this is most confusing.

Can anyone tell me how PostProcess motion blur is different than the default you get in any rendered sequence?

is your camera inside the post process volume? or is your post process volume set to unbound?

Yes to the latter.

PostProcess volume is override of default settings, but there must be some defaults for engine to know what and how to actually render. One of those defaults is that Motion Blur is on. Most defaults are defined within the .ini files, but the most used subset of them is also exposed in the Project Settings, under rendering section: