Why does diff output "Common subdirectories"?

Question

Data Comparison

Why does diff output Common subdirectories (Common subdirectories directory1 directory2).
Directories with different names, but should be with the same content and you need to find out.
In theory, he says that the directories are shared, i.e. the same, however, with the -q (--brief) option, the output is still the same. Tell me why?