`freebsd-update upgrade` looks at `uname -r` instead of `freebsd-version`. When running in a jail, this results in the error "freebsd-update: Cannot upgrade from 10.3-RELEASE to itself".
This can be worked around by setting UNAME_r=`freebsd-version`. However, this is an unnecessary complication, and there's no clear path to discover this since one must look in the uname man page for problems with freebsd-update.
The "--currently-running" also exists so one could do "freebsd-update --currently-running `freebsd-version`" which is decently documented.
(In reply to Gerald Aryeetey from comment #1)
True, thanks, that's a better work-around. Still, it seems obvious that it should default to what's actually installed, which is easily determined by `freebsd-version`. `uname` is often not what's actually installed.