I spent a career with instrumentation. Accuracy is one of the most misunderstood, misused, and abused word in the world of electronics. Never trust a salesman or his spec sheets. Never accept an accuracy figure or specification without knowing and asking a whole lot of questions about how they are arriving at their specification and under what conditions.
It's a very interesting field, measurements and standards, but far from as simple as one might first think.
Lefty