The basic method for hardness measurement is using an indender and a standard force on a material surface and then measuring either the depth or the area of the imprint.
However, is happening with almost all hardness scales is that you cannot obtain meaningful results for all types of materials, for a given force and a specific indender. Therefore what you are doing is you change either the force level or the indender.
In the case of Shore A, and D you can see in the following image,
the indender for D, is more sharper compared to A, and as a result it can more easily leave an imprint on harder materials. On the other hand A, will leave a moderate imprint.
The overlap region is basically a good way to validate to make sure that the interpretation of the results from both methods is properly normalized.