Number of craters in one square Kilometer

Why do you study the number of craters in one ?. You will now with following example:

We study the following photos:

           


We compare these photos, and we observe that in the first photo there are more craters than in the second photo. What can we conclude?

The first photo there are more craters than in the second photo, but we don´t conclusion nothing, because in the first photo the surface of the Moon is greater than in the second photo.
So, we will study always the number of crater por in all photos of the surface of the Moon.