Fluid dynamics is very important in film development. Under agitation causes the film adjacent to edge to develop slower. If that edge is a sky value, it will exhaust the developer at a slower rate than the rest of the film. This is not just my observation but the observation of every pro I went to school with and had to put their butt on the line when they saw poor results from using the manufacturer's recommendations with regard to proper agitation
Shadow areas exhaust the development more quickly. I have been using 6 inversions in 10 seconds once a minute ever since I began to understood the concept of fluid dynamics and flow. It is most important to stop the vertical movement of the reels inside the can, as I detailed previously using a small length of PVC pipe.
I conferred with many photographers before the internet and we compared notes and found out the same thing. You can eliminate the small tank (30 sec) vs. large tank (60 sec) intervals, by adjusting the processing times for smaller tanks down slightly.
Consistency is everything. It is the only way to eliminate variables. It is also crucial to mix developers using distilled water. Either too hard or too soft water will wreak havoc with your film. Too soft can cause the emulsion to lift right off the base, too hard can cause mineral deposits to form during development, creating spots that print black on the film. I had film ruined that was used during research for a Guggenheim Fellowship I was awarded back in 1972 due to soft water.
I banged my head on the wall for quite some time understanding all of this. I hope this helps. I've had nothing but consistent results since 1976, when me and my photographer buddies met to discuss all of this and figure out both the source of the problem and an approach to remedy it.