You discover 100 marbles, all the same size and pattern. Some are red, some are blue, and some yellow.
You decide that the marbles are functionally identical, with color being only a superficial difference.
Bob takes a keen interest in the marbles and begins to experiment with them. He finds that, on average, the red marbles roll a little bit further than the blue marbles when the same force is applied, and the yellow marbles a little bit less.
It must be measurement error, you decide. After all, they are all marbles.
But Bob is stubborn and repeats his experiments many times, and stands by his methodology and accuracy. He wants to understand what's happening here.
Eventually Bob does a detailed composition analysis on the marbles and discovers that each color has a slightly different mineral mixture. Red marbles are slightly more dense, and yellow marbles slightly less.
Bob thinks this probably explains the experimental differences.
You decide to fire Bob.