I think you’re right. From my experience, it’s definitely looking like it’s on purpose, however, the tolerances are slightly different on each phone.It seems to be the same case for the titanium Watch Ultra and intentionally designed that way. Someone mentioned it's meant to keep the glass from breaking when absorbing impact from a drop or due to the thermal expansions (extreme hot/cold) of the metal frame, components, etc. It's normal on maybe all phones to a certain degree, some more noticeable than others and those who don't notice aren't as OCD as Apple enthusiasts on MR/Reddit. The tolerance is as tight as it can be.
A shame, since, when I take both my 14 pro max and 15 pro max in hand, the 14 pro max seems ultra smooth in that area.