In real numbers the atmospheric pressure will be multiplied by the compression ratio. Sea level pressure under standard conditions is about 14.7 PSI. Multiply that by the compression ratio of 6.5, and you get a theoretical pressure of a little over 95 PSI for a perfect engine. It can go no higher...and engines are rarely perfect.
Sometimes, a combustion chamber is reduced in effective size due to deposits, so the reading can be a little higher.
I think gauges sell better if they read on the high side, so gauges are most useful for comparing the cylinders to each other.