I wouldn't use "correct" or "bugs" in case of tiny deviations like 1–5 of 255, as you point out, there seem to be different algorithms behind – while all involved software (OS, GPU, profile(s), Affinity or PS) would use the same ways to achieve their goals in a "perfect world" only. Just FYI … at 0:28 min Apple's "Digital Colour Meter" checks the values, with different profile conditions: tiny deviation for "Native" + "L*a*b" but confusing large deviations for the three different RGB option