It only does that for the FreeNAS-9.3-STABLE train (for now, anyway).
There isn't a particularly good way to find out what's changed for the nightlies; the ChangeLog is generated semi-manually, so it's only done for the -STABLE updates.
This is pretty much as intended from the beginning: the nightlies are just that, built nightly, mostly automatically, and without any checking to see if anything is fixed or added.