To claim otherwise, that film is always consistent, is nonsense. Too many variables.
Hi, this has not been my experience with the lower-speed pro portrait/wedding films from Kodak. The last of which was Portra 160. I have had first-hand knowledge (some years past), based on sensitometric tests, showing that they behave nearly identically, even across emulsions. When I say nearly identical, I mean that if I were to present you with one or two dozen tri-color density plots and said, try to find one that is different, you wouldn't be able to do it. Except that very occasionally there might be some sort of running change in the product, or perhaps one emulsion with a very slightly different sensitometric signature.
The outfit where I worked was a large chain studio operation, with a large processing lab. We ran several miles of these films every day, printing drastically more than that. Because of some long-forgotten problem we began screening every new emulsion number via sensitometric wedges. (The complete emulsion runs were reserved for our use.) Our warehouse would pull a couple of semi-random 100 ft rolls for us (QC Dept) to check. It would NOT be shipped out to studios until we gave the ok. (This was probably 1980ish.) I don't recall exact dates, but I would say by the early 1990s almost everything would be identical.
We initially thought that the entire web of a master roll could not possibly be consistent. So to see how bad it actually was we pulled a number of samples (100-ft rolls) from certain locations in the film web. So a set of cross-web samples near the beginning, middle, and end of the master roll. Maybe a dozen or more cans of film. We exposed a handful of sensi wedges on each sample, then processed the film. To my great surprise the plots (full curve, tri-color) were again near identical. (There is always a slight amount of "noise" in a plot, but most points would be within 0.01 density units of a reference, or sometimes maybe 0.02. As I recall; I'm going by a somewhat fuzzy memory now.)
So, my faith in all the so-called experts whose writing I had previously trusted was completely lost. All I could conclude was that they had never done the tests, just presuming what they said was true.
I found, multiple times over the years, that other certain parts of the common wisdom were also wrong.
Now, it's possible that the motion picture part of the business (in your wheel house) has greater variations in their film production. But if you are seeing significant variation, and it is a professional Kodak product, my guess would be that either the processing was not consistent enough, or perhaps the lighting varied. I will say, that in our lab, we had extraordinarily good "process control." I was the QC manager for a number of years, staff of about 5 or 6 people, including a full-time chemist. We oversaw most aspects of the chemical mix operation and process control, and I was intimately familiar with most aspects of that.
I can't say anything for certain about other films than the ones I specifically described, but in those cases I'm certain - the film was extremely consistent.