All tyre pressure gauges are inaccurate, in my experience.
Most rally guys carry gauges in their cars. We compared a few of them before the (delayed) start of a stage once - none of them read the same as each other - most were within a few psi (which is a fair bit, in percentage terms), and a couple were several PSI out. Interestingly, the newest gauge was also a big dollar one, and it was clearly miles from accurate...
There was a lot of talk about which gauge was the most accurate, but we'd all learned to compensate for our own gauges - it certainly explained why two blokes running the same tyres on similar cars were most happy running apparently very different tyre pressures.
So... my take on it is to buy a quality gauge and stick with it - and recognise that the numbers only strictly relate to your gauge.