The gun only gives you the closest formula match - contrary to what some believe (that it will create a formula where none exists in the system). I used to have painters call me to bring it out, even when they already had the right formula (and they didn't want to mess with matching it).

As for why one formula won't be a perfect match in all instances ... Sun and weather can play a role, but with modern paint, not as much as in the past. You can take 10 brand new cars with the same exact paint code, and have 10 different variations.

One reason is, it might not be the same paint. The OEM might buy paint from different suppliers, in which case, it may or may not match. Even different batches from the same supplier may not match.

Another reason is, the parts might be made by different suppliers (who each use different paint suppliers) - things like door handles, strakes, window surrounds, bumper covers, all of that kind of stuff is usually made by someone else and shipped to the factory, painted, for assembly.

One of the most interesting reasons has to do with metallics. At the OEM facilities, the paint is often a considerable distance from where it's sprayed, and it travels through pipes. Every time it hits a 90° bend, those flakes are slamming into the pipe - which is exactly how you make them smaller (in a big rock tumbler type thing). So ... paint matched perfectly to the standard, by the time it gets through those pipes, isn't going to match anymore.

When you get into silvers and whites, in addition to the standard formula, there are almost always a large number of alternate formulas, some of them very different from each other.

Finally, the documentation from the OEM's on anything other than the body color - trim colors, interior colors, etc. is usually very limited, and generally not well documented or researched.