Question about phosphors

[Rinehart]

I have an article from the Radio & Television News for October 1951 about how to build your own colour wheel and synchronize it so that it would be able to reproduce the CBS colour signal. In the course of the article, the author mentions certain problems you will encounter, amongst which is the variability of phosphors: "as a service technician knows, it is rare to find two phosphors that have the same colour." He doesn't explain what he means by that; does it mean that there were many different varieties of phosphor, or that because of some aspect of the manufacturing process, there was a lot of variability in the emission spectrum of a single kind of phosphor--ie, it was hard to control the outcome of the manufacturing process--or does it mean that phosphors decay in some way and slowly lose their original emission characteristics?
I have attached the first page of the article.