It's not just a matter of visual acuity. As we all should have learned in school, the retinae in our eyes have two types of visual receptors: rods, which are strong light receptors, and cones, which are strong color receptors. The rods and cones are distributed in such a manner that when we look straight ahead at an object most of the light strikes color-sensitive cones. But most of the light entering through our peripheral vision strikes the more light-sensitive rods. You can see many more stars in a given constellation by looking at it around 30° askance instead of head-on. Avid stargazers know this trick and will, thusly, report more visible stars than someone who doesn't know it.