> Is this done because of the error in weighing the smallest parts, so they have a margin for error by allowing for an extra 1 or 2?

This is a secondary benefit, the primary benefit is if the end user loses/breaks one. That part very well could be show stopper (Ikea 110630 anyone?). Now the end user is stuck - has to call, you have to ship, do you charge? do you give for free? they have to wait. they're annoyed, you're annoyed.

No one is happy.

The supply chain headaches for giving exact number of tiny parts is terribly expensive, relatively speaking. So you give spares because in the long run it's way cheaper.

Just tacking on to mention the smallest parts are most likely to be lost, they’re the ones that - if dropped - seem to bounce and roll under a refrigerator or into the ether. They don’t give extras on the larger parts because they’re not likely to be lost. Frequently enough all it takes is a violent/careless bag opening to send the small pieces flying.