Thanks! I have considered an FAQ along the lines of “How power meters lie… let us count the ways.” 
But seriously, we have the repeat post of people reporting that their new power device doesn’t match their current one. Usually after minimal testing in less than ideal circumstances. Power meter type, location, setup and more are all factors.
Doing a quality comparison is more work than most people want to do, so I’ve not done more than make one real FAQ linking to some power data comparison tools. I should probably add to that and touch on the most common issues and reasons that devices might vary from each other.