Perceived paint color is due to the reflection of light that reaches our eyes. Both wave length and intensity contribute to our perception of color. Unfortunately the light source is also a critical factor. If a wave length is not present in the light source it can not be reflected. Also a factor is the surface as wavelength(s) can shift and intensity reduced by imperfections and how deep the light penetrates before reflected. If the light does not reflect back to the observer or instrument it will not be a factor in the perceived or measured color.
A spectrometer is an instrument that measures wavelength and intensity. They can be very accurate in the measurements but still due to the above factors not result in a perfect match.
The library of color matches was determined under a particular light source with the meter a given distance, color samples specially prepared and likely no other sources to provide interference. The scan of a painted surface by the local paint supplier does not have the luxury of those controlled pristine conditions and therefore, can be not be expected to be 100% accurate. Given all that, the matches more often than not can be more accurate than our eyes.
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