For requests without a version, all mappings are a considered a match. After the sort, the mapping with the highest (most recent) version is selected as the best match.
In a scenario where versioning was introduced not from the beginning but after some time, it makes sense to match clients that don't send a version to mappings that also don't have a version.
The same can be achieved by setting a defaultVersion in the configuration, but improving the mapping in this way would not required that to work as expected.