Perceived volume compensation a bit, roughly? Console emulation style? Versus digital math perfection? Don't know exactly. But this is what may differentiate among DAWs summing engines..... Also 32 bit float vs 64 bit float accuracy, PDC compensation (milliseconds delay, causing other phase summing etc) and other details.... Audio engines maybe not the same due to some algos...
I've been using the 3 dB pan law in my stuff - it's most common and pretty close to a circle. I think you're fine with 3 dB or 4.5 dB, those yield the least 'wtf' results. Avoid 6 dB and 0 dB 'no-law' laws. Those make it so that you're panning along straight lines instead of a round circle. Like strafing left and right in a videogame when you need to turn your head.
Do you do any "dynamic" panning, automation or such? If not, don't worry about it, your ears will set the stereo image for you. But anything moving then you have to try things out not to have things jumping from side to side too harshly. 3dB should do the trick. m/s then pan is another thing but best not to mix these up too much unless you know what you're doing.
