Remember the accuracy of the gauge too, I'd bet most the ones used are just naff cheap ones like mine
So the components used will have poor tolerances and variations in results, so if you're a few PSI/0.1bar' off someone with similar/ same mods then i'd put it down to that.
Best way to tell is do an identical drive in the other car and swap gauges, see if you get the same results in both. Even then there's no guarantee that either is correct, they could both be off :p