Query for radius appears to not be accurate

Hello. I've been doing queries within a radius where I start at 10 miles and then I increase by increments of 10 miles. What I've noticed is that locations that should be within my radius are coming back in larger radiuses....for example 1 location within 14.3 mi and another within 17 miles are not getting returned at the 20 mile radius, but instead at the 30 mile. And 1 location within 25.2 miles and another within 28.5 are not getting returned until the 50 mile radius instead of expected 30 mile radius. I verified the distance from my location at two websites that calculate distance between coordinates. Has anyone noticed this?