That is why you should not use a computer with a mechanical keyboard on an airplane without the stewardesses permission ...
Anyway,... is this a comparison between black and grey single-piece Model M keys? Perhaps they are of slightly different types of plastic, with different amounts of dye. A plastic could have its own colour, in which case the amount of dye has to be measured so that it changes the colour relatively. The more opaque the plastic is to begin with, the more dye is needed. Or perhaps some other additive has to be used to accommodate the dye.
