Speakers typically average around 10% THD and their responses are all over the map, if you take off the smoothing tricks they apply when testing them.
Move it 2 inches and sweep it, it will be all different again.
Pound it for 6 months and test it, it will change.
If you need a super close match, only removing the acoustic enviroment will do that. That means electrostatic headphones is the solution.