Electronics is a sum of errors. It is doubtful that any two parts will ever be exactly identical in every possible respect.
When you put them all together, you hope that the end result will fall between acceptable limits.
If you carefully test a dozen or a hundred "like" devices, you would come up with an "average" performing unit...... what you would normally expect for performance.
You could also find a few really "hot" ones and a few "dogs".
Just to look at coils of the same type.......
How well is it tuned for resonance and bandwidth ??
How well is phase cancellation achieved ??
How much gain does the preamp have ??
How quiet is the preamp for noise ??
Adding it all together, it would be possible to have two coils with consecutive serial numbers that are quite noticeably different in performance.
Of course, there will be some differences in how well any particular control box handles the received signal.
That should provide somewhat of an answer to part of your question.
You will get many differing opinions on coils and detectors. You never know if one was "hotter" than average, or the other was a bit "doggy", or both.
Many times they will not be tested side by side, so you don't really know if it is really a good comparison.
Have fun with the confusion.
HH