A merge between two branches, where another set of files has been changed in each branch, is a simple (or "trivial") merge. Git does not need to look into files to see what to do - it just takes the last one.
However, if both parents of a merge modify the same file, then Git should look inside the file to see how to apply both changes. This is a "merging level of content." Results can be applied automatically if there is no conflict or manual resolution may be required.