Checking pressure will always result in a pressure drop, no matter what you use to check it with. I have a small air compressor that runs on both 12V and 120V. It has a digital readout. I use it to check pressure. I put a couple psi too much in, because you loose that much when removing the hose.
The same thing happens when you pour oil, especially thick oil, like gear oil, into something. If you use a measuring cup, and fill it up to the exact amount, then pour it in, quite a bit of the oil will remain stuck to the inside of the cup and funnel if you used one. You have to compensate for that with a little extra oil. I'm so picky about it that when I mix up a gallon of 2 stroke premix form one of those little bottles, I pour some some gas into the "empty" bottle, then shake it, to get the oil out that stuck to the inside of the bottle.
I used to work on A/C units, and knew "professionals" who just couldn't wait to "throw the gauges on a unit" This is a serious mistake. Not only are you going to lose some refrigerant, you also risk contaminating the system. Gauges should ONLY be used when it is obvious they are necessary.