The single PID design is sending the exact same power to both heaters based upon what the thermocouple in the top plate is detecting. Because of this you may see a minor difference in temperature using an external thermometer like a javelin probe meat thermometer or k-type thermocouple. I don't really recommend the IR guns if you're looking for accuracy, they tend to vary based on distance and I've verified inaccuracies using multiple different thermometers/thermocouples, YMMV.
In the grand scheme of things, these temperatures fluctuate on the plates surface naturally as the heaters are pulsing heat at the core of the plates. It's not uncommon to measure the temperature on the plates and see it change 1-2F towards the center of the plates, and perhaps even more towards the corners.
I agree that most rosineers are not adjusting both plate temperatures, but perhaps in their situation where their top plate is attached to their shop press, it may be heat draining without insulation layers, so one may need to offset the temperature on the plates by increasing the temp on the PID.
Luckily in the case of these AIO units, they plates are well insulated with 2 layers of material (FR4 & bamboo) which really do a good job protecting the unit from heat draining.