Hi Bill, any color is consist of a few toners that makes that color, so if the mixer is not precise in mixing the exact amount the color can be off.
A color can also have a few variances, a variance sheet will give you the closest match. A good painter can add a few Milligrams of different toners to get a blend able match!
Now silver Gold colors are different story, for a door dent repair we had to paint the whole side to get a match, major toners of Bristol is white and week black, hope this helps