Natural variables? What the heck could those be?
1.. Is YOUR search coil 'in spec' or are OTHERS search coils 'in spec'?
2.. The internal Ground balance trimmer would have to be set
exactly the same for YOUR unit and ANY BODY'S unit because if the GB is 'off' too much, the results will/can vary.
3.. If relying on only the speaker response, the variable would be how good is YOUR hearing compared with MY hearing or HIS/HERE hearing?
4.. What type of audio response do YOU call a good 'beep' compared with what I/THEY might consider a good 'beep?'
5.. What coin position was used when swept across the search coil?
6.. Was the target swept across the center-axis of the search coil?
7.. What sweep speed was used to do an "air test" with the sample target?
8.. Are actual measured distances used, or just a 'best guess' distance?
Those are a few air-test variables to keep in mind.
deathhare said:
Yes, of course...stock coil here.
I got these numbers without headphones:
7" on a quarter
6" on a nickel
6" on a dime
6.5" on a copper penny
7" on a zincoln
12" on an iPhone. 
These are the points when I got a signal that I would have dug, not some weak little crackling chirp that sounds like trash.
Another 'natural variable' would be trying to compare in-ground depth performance. Ground mineral conditions can change, even abruptly in some really challenging sites. Then the best comparison tests would be on long-lost, naturally-located and carefully recovered targets so you can know the exact position and measure (accurately) the depth from the search coil height to the located target.
Again, ground conditions can vary. Your detector might not be optimally tuned for your ground, or someone else's might not be for their ground, either. That can effect your results as well as others. A too negative GB will cause a lot of falsing in the discriminate mode. A too positive GB can cause a loss in detection depth, or total loss of detection, of higher-conductive targets, especially things like big silver $1 or some 50