From: Eric Walker
>> … How can it be when quarks have variable mass? > Variability in the mass of the quark does not prevent an accurate proton mass > from being specified. What it does is places a bound on the numerical > precision that an accurate proton mass value can have You still may not have an accurate understanding. These are real differences - not a function of numerical precision. Of course, quark variability places a bound but that bound is comparatively huge. Hydrogen extracted from deep old methane can have different average mass than hydrogen split from rain water. Interstellar hydrogen or solar-wind hydrogen can vary markedly from either. The source is important. There is no other way to accurately explain the history of variation in measurements. This is not about numerical precision of an instrument so much as it is about unknown variables and the past 13 billion year history of the sample.
