Did you order that particular model? Does the documentation say that it will work with 6 cylinder engines as well?
If you look to the right side of the face there are pairs of scales, one for "6cyl" and one for "8cyl". For "4cyl" you double the reading from the 8cyl scale. You could probably read a 5cyl engine too, if such a thing were to exist (5 cyl with a coil), but you would have to multiply the 6cyl scale by 6/5 or the 8cyl by 8/5. The meter is essentially counting how many spark fires in a minute. The different scales effectively divide by 6 or 8.