A member of the MAME Discord server just brought up an interesting issue, and we're both a bit stumped as to what's going on here, what the underlying logic is for marking a system as Available versus Unavailable, and if this is either user error or some poorly-documented filtering logic. I confirm the behavior, so does a third user.
Here's the Available filter for the first user - 'nes' is there, 'nespal' isn't. It's not a simple parent vs. clone thing either, as both j5nite and j5nitea are listed:
Meanwhile, the other user with a full 0.239 set (minus some CHDs) provided this screenshot. The arcade machines that are missing CHDs makes sense, but what's with the computers?