Q: Why does the accuracy of a hand-wound watch vary from day to day?
A: It's not much of a problem now, but up until a long time ago, it was common for the accuracy of a hand-wound watch to vary from day to day. The main reason for this is that the torque of the mainspring, which is the power source of a mechanical watch, is not constant. When the mainspring is fully wound, it has a very strong torque, which then remains flat, and then the torque drops sharply just before it unwinds. For this reason, the accuracy of a watch changes significantly between when the mainspring is fully wound and when it is just about to unwind. This was a major problem with old mechanical watches, which only had a power reserve (running time) of around 40 hours.
To prevent this, some luxury watches are equipped with a "stop winder" (a Maltese cross) to stabilize the torque of the mainspring, a fusée-and-chain to pull the mainspring, or a constant-force or remontoire. Furthermore, in recent years, mechanical watches have tended to reduce torque to extend their power reserves, meaning that accuracy is no longer as susceptible to the strength of torque as it was in the past. Mechanical watches with a power reserve of three days or more can be considered to have relatively stable accuracy.
However, for watches with short power reserves or older watches with basic designs, accuracy can still be affected by the strength of the mainspring's torque. If you want to stabilize accuracy, it's a good idea to wind the mainspring at set times. Also, for watches with a power reserve indicator, it's recommended to stop winding the mainspring slightly before it's fully wound, as this tends to stabilize accuracy.
