system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can some watches be more accurate than others?

 I don't know much about watches, but I remember reading some watches only make a few seconds mistake each day, whilst others have larger error range. Isn't there a mechanical system that keeps the thing automatic? How can that create errors?

Also some watches can remain accurate for decades, if one watch maker can do it, why can't other do it?

I guess I'm just confused as to why there are errors and if there are errors, why some brands have larger margin of errors.

Thank you in advance for your answers.
gpt
Quality control and tolerances.

In any machining operation your parts have tolerances. A tolerance is how far out of the specified dimensions the part can be. Very exacting tolerances get rapidly more and more expensive, as they require much higher quality of both manufacturing equipment and labor, and a quality control system that is willing to throw away potentially very expensive parts for being a *tiny* bit too big or small.

A mechanical watch with very tight tolerances will be very expensive to make, but it will be extremely accurate because you know to a very high degree of certainty *exactly* how all the parts interact.

A watch with loose tolerance will be much cheaper to make, but won't keep time as accurately because bits and pieces are going to be a little bigger, or a little smaller, or a little heavier than they should be, and in unpredictable ways.