Generally it is hard to say how many sheets of paper one can process with a certain amount of fixer because not only does it depend on paper type, fixer dilution and stop bath carry over, it also depends on the amount of undeveloped silver in your images (think high key vs. low key image matter). You can fix a lot more AgCl than AgBr or AgI, so emulsion type (warm tone vs. cold tone) also makes a difference. If you have developer carry over, you might introduce potassium ions which supposedly have a detrimental effect on fixing. I would assume that published numbers on fixer capacity are conservative estimates that cover most cases of regular dark room work, and that retained silver tests are to be used by those who want to be 100% certain.
And about Kodak color fixer with its pH of 6.5: the reason this pH is chosen is because dyes change their hue if pH is off, not necessarily because this pH is ideal for fixing.