Parallel resistors, greatly imbalanced in value, can be for trimming. If 1% at 1,000 ohms, then 1MegOhm in parallel will reduce the total by 0.1%.
Parallel resistors will have a greater area, thus more vulnerable to electric field aggressor flux inflows.
Parallel resistors may have more underlying Planes, to which HEAT CAN BE DUMPED through the insulating epoxy-fiberglass substrate. FR-4 has about 200X the thermal resistance of copper, but thin sheets (1/16 inch, 1/48 inch, etc) are the distance of these PCBs.
Parallel resistors may be needed to reduce Thermal Distortion, where for Audio material (or music) the bass notes will greatly modulate the RESISTANCE and likely change the gain. This gain change will reflect onto the high tones, as AM sidebands.
Read work by Walt Jung on sizing resistors, to reduce thermal distortion that degrades Power Audio Amplifiers.
The thermal time constant of 1 cubic centimeter of silicon (clay? ceramic base of resistors?) is 114 seconds.
The thermal time constant of 1 cubic millimeter (about the size of a SMT resistor) is 100X faster at 1.14 seconds.
The thermal time constant of 100 micron cube of silicon (perhaps the size of a large resistor on the surface of an Integrated Circuit) is 100X faster at 0.0114 seconds.