I have always monitored tyre pressures of my cars with consideration to safety and economy. Lately however, I am continually baffled with the readings that I record having inflated tyres with a preset electric pump when comparing with a small hand held digital reader and a manual 'pen'.

The two hand held devices always register less than the preset pump (never the same reading either) and are at least 10% under if not more. I have avoided forecourt tyre pressure machines due to their reputation of inaccuracy.
Should I accept the pump has measured correctly or is there a way to obtain an accurate tyre pressure, any thoughts?