True, I always check the zero and clean the measuring surfaces before every use, None of mine have ever been off much. I think its me just looking at them wrong, and its always splitting hairs as to what is perfectly zero.
Now I can say this. I have noticed that on some of the cheap ones the sharp tips may not be perfect, I only use that part for measuring bullet crimp and all i am looking at is a difference so it does not matter. I use the Flat parts to measure everything.