You are doing your measurements qualitatively using the Mark I Eyeball which in your case is now over 80 years old IIRC.
I did mine with the IB sensitometer and the Macbeth densitometer and did it qantitatively.
I trust mine, and I trust the differences that I have seen. Papers can sense as little as 100 milligrams difference in bromide or chloride concentration in a liter of developer. It affects speed and curve shape both. In some papers, it affects fog as well.