My guess is that they compute the resolution at which the MTF curve drops at 50% as an indication of sharpness. In one they might use LP/PH (Line pair per picture height) and in the other one LW/PH (Line width per picture height). That explains the ratio of 2.
But it is nice to show these graphs. They are the reason why I don't like to Fine Tune any lens just taking a picture of a target. The curve is too flat for this method to be accurate, and a tool like LensAlign is way more accurate.