With respect to Android, I was under the impression that (in part because key members of the Linux community are allergic to the notion of a stable device driver ABI) the specific release of linux used on a particular model of phone was typically frozen at some point before the time the phone shipped, and subsequently only patched as needed.

The mere fact that the source is available doesn't mean that someone will take the (potentially unbounded) time to port it to a future release and test it sufficiently.