It looks intentional, either from the solder paste stencil or by hand at some point afterward. I doubt we will be able to figure out why it was done.
Adjustment can compensate for variations in several different components at once, and sometimes do a better job of it, so I suspect if they could have just used 1% or 0.5% resistors and gotten acceptable performance then they would have.
Modern designs often use EEPROM storage to save calibration settings, but usually there is a compromise involved (for example you might lose 20-30% of dynamic range to ensure that all units can be trimmed in despite production spread) and there is the possibility of memory corruption "bricking" the device.
Trimmers can drift, but if the circuit is well-designed and the adjustment range is no more than necessary that is not usually an issue, assuming screwdrivers are not involved.