Hannas aren't known for reliability. One of these days that company is gonna sue my ass, lol...
Liquid testers are accurate and reliable. They don't read to like 2 decimal places, but this isn't an exact science! As long as you can tell it's in range you're fine.
Bottled drinking water is not good for plants. I bet your tap water isn't NEARLY as bad as you think it is. Grab a $10 liquid test kit from the aquarium supply shop and see what the real numbers are.