You said you adjust the meters yourself. Are there any tutorials online that I can learn to calibrate them? Is it something that a beginner could do?
Yes, adjusting inexpensive meters is easy and is something anyone can do. If you open them up you will usually find a trim pot inside marked "DC V" or something similar. You need a reliable voltage reference such as the
DMMCheck or another meter of known accuracy. You just adjust the trim pot until the reading matches expectation. (Careful with the adjustment, it only needs tiny movements.)
The advantage of inexpensive meters is you can do this adjustment without much fear, whereas you wouldn't dare touch the adjustments on an expensive meter like a Fluke or an Agilent.
When you play with meters you will find that 3.5 digits of accuracy is stable and readily achieved. It's the 5th, 6th and more digits that start becoming a challenge. Luckily most inexpensive meters don't have so many digits to worry about, and neither do normal circuits care about such accuracy.