Using inaccurate apparatus for results, is just like questioning engineers, how do you prove that it's practically correct?
Shall I invite you to, visit with your gauge and use my gauge and then we will take 6 set of readings, plot a graph, take the gradient and using some pressure laws and reach to the conclusion that you are using inaccurate apparatus? Do you want to do this or getting a fuel pressure gauge and sharing us the readings Correctly and enabling us to help you would be a good strategy?