Richard,

Yes, that does it..! (as per your findings)

But you must have missed the vital bit I said about Gamma.

Goto your colour prefs, and where it says " Gray : Dot Gain 20% ", change the drop down menu to " Grey Gamma 2.2 "

Then try it, and I think you should find no problem. Values report as per expected.

The way I see it, is that dot gain 20% is not a match for the RGB gamma, therefore mid point grey in greyscale does not corroborate with mid point grey in your RGB space because the two are not the same. (However I can't account for why 62 is filled as 61...)

But try it in Grey Gamma 2.2... it works here