You are not alone in your assumptions...not at all! Many of us assume precision and accuracy when it comes to an analytical test...but reality is often different!I guess I'm just assuming the equipment is sensitive enough to take those measurements and do it with some degree of accuracy.
No idea, but I always thought that the point of the ICP testing, provided it was done correctly on an appropriately calibrated machine, was to get a highly accurate, laboratory grade analysis of your water on items we cannot test for with hobby grade kits and confirm the readings on the things we can test for.
I guess the range they're using is subjective.... And clearly the tank is rapidly using the individual trace elements I'm individually dosing on a daily basis when I see a "not detected". I interpret a "deficient" reading as my daily dosing dosing is not high enough to get into their suggested range. I think the whole point of keeping the water "in range" is to make sure there is enough major and trace elements to keep the tank balanced and properly "feed" the inhabitants at all times. Should you have a non detected or deficient reading on an element you're dosing, my assumption would be that you are limiting growth or coloration as the corals would be using what you put in, and then eventually running out of what they need if you can't consistently keep the tank in range.
FWIW, this is all based on assumptions and I have zero scientific background. To put it simply, I have no idea what I"m talking about