That's not necessarily correct George.
A vast majority of detectors will not air test with great depth. But, will go deeeeeeep. The reason being, most detectors need to pick up the "matrix" or "halo" around the target while IN THE GROUND.
Heck, I've seen the better White's models and Minelab's air test horribly. And we all know these are VERY capable lines. But, get them out in the field and that's a different story.
Air testing is completely different than actual in-ground performance on targets.
Try it out one day. It doesn't make sense, I know, but it's true