I own 14 brass ones they are all with in 1% of each other and can very easily be calibrated, I do so every 6 months. I have a bunch of digital ones that are all manufactures and run from 18% to 99% when placed in a 70% environment. they may start ok but quickly go off. Digitals that use a resistive sensor go off quickly and are very fragile they do not handle dirt or dust well. The accuracy in my experience of digitals is only good for 6-8 months then it is hopeless (beyond 10% off) I save a ton of cash with calibratable analogue. The digital units that can be adjusted are costly and the dial easily moved I have found the two I have to be fine in the evening and completely incorrect in the morning. I believe my snakes in exploring 'adjust' them.

I calibrate and check every unit every 6 months and it is rare for me to need to adjust more than 3-5% the digitals are all over the place and the test kits are expensive.