some its just the stickers location, some i think its either inductive delay or the scale on the sticker is wrong and each degree gets progressively farther from right. Ive seen dial back lights as much as 4-5 degrees different from my msd light. If you arent spraying the shit out of something, or running a pro level deal, id hope they all are giving a repeatable answer and once you find the happy place for you, it should be fine The dial back type lights are all over the place, but im sure that each light is consistent to itself, so if you use the same light every time, you should get the same answer.

If you need accuracy and consistency from one light to another, and your stuff is on the edge where a degree too much may hurt it, ive tested probably 20-25 MSD self powered lights against each other and they are dead on each other.