I worked for a summer in a textile plant in the late 70's and all of the dyes there were mixed by hand. There were little receipes for each one with differing amounts of various colors used to create each shade, not unlike how paint is mixed at your local Home Depot. Different sized ladles were used to scoop in amounts of each contributing color but I can tell you that how much ink was in that ladle would vary scoop by scoop, sometimes dpepending upon who was doing the mixing.
I can easily see the American Litho inks being subect to less than precise mixtures, resulting in sme natural variance. Whether they were mixed prior to printing or during printing I can't say but if it's the latter then certain components could run out just before the end of a press run (or be used for the very beginning of another run).
Just a thought.
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