That makes sense... So this is essentially a downgrade to JWR66N, but with a JSS build name and the changes/improvements in 15J?
I don't have access to their source tree, so I can't say for sure but I suspect they went back and looked at the changes in the multi-touch area and either backed out the changes or saw that they could tweak things a little more to improve beyond JWR66N levels.
Given the naming of the release I would guess most of the release is the same as JSS15J with some isolated changes to address issues.