Originally Posted by brentc
Although it may not always show a consistent drop in MV, it certainly doesn't show a consistent increase. When the data is analyzed it's either a very slight increase or decrease from the previous powder charge, indicating the plateau, or more simply it shows the point of diminishing return where more powder is gaining you very little or no performance increase with the increased pressure and brass wear.
I think diminishing returns (plateau as you put it) was MikeCR's point.
With enough powder, you could start setting your lugs back or experience case head separation which would probably cause a statistically significant decrease in MV provided you live long enough to collect enough samples.