I'm sure what you'll find is there is a percentage off... not a pressure amount off. For example, if you have 100 psi in the tank, the gauge will not show -100. So if you are 200 off at full, you will probably be about 100 off at 1500, 50 off at 750, 25 off at 375. So really, it becomes relatively insignificant since zero is still zero. But a fluctuation of 100 psi or so at full is not uncommon.
Of course if it bothers you...
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