It's done that way because of distortion of the image from your phone camera. If you calibrate 1" way over at the edge of the picture, and expect the app to use that to accurately measure a group in the center of your picture, the measurements will be way off. That's not a fault of the app, that's the fault of the phone camera.Sorry for a late reply but I have used both recently. My only issue with Ballistic-X is that you can only move so far from the center of your image to set the end points for your 1" distance. It seem you've got to have a ruler benchmark, whether it's an actual ruler or the grid lines on the sight in targets, within 2 inches or so of your grouping. Probably done to help remove distortion, but if you're shooting the center of the sight in targets the grid lines are too far for you to utilize as the 1" reference point.
Obviously the user needs to work within some of these limitations a bit, instead of expecting an app to ignore reality. Same for the complaint about multiple groups on one picture - take a separate picture of each group with the camera centered over the group if you want it to be accurate.