This may be related to the size of the lens resolution test chart you are using, and the interpretation of the lp/mm numbers from the photographed image of that test chart. For the ones I have, the readings are designed to be accurate when photographed at a distance of 101 times the focal length of the lens from the film plane. For a 50 mm lens, this will be 5,050 mm, or 5.05 Meters, or 16 feet and 6.82 inches. Making a test chart of a precise accurate size is one of the reasons why they tend to be expensive. The best ones probably are the ones made on a glass plate. They do not change with variations in humidity as is possible with some of the paper ones.