does this have any significance? Can any AV guru comment on this pls?
I'm no AV guru but I have a theory...

Depending on how the AV is setup by the maker (or if adjustable, by the user (Trend Micro used to have this capability)), a scan only drills down so far. Your test was 3 archives deep. My guess is that if you tested only 2 deep, the number of hits would rise by a fair number (perhaps all of them??)

It all comes down to efficiency of time and computer resources. The longer scans take, or the more CPU cycles they consume, the poorer computing experience the end user will have. Plus, it should normally be safe to assume an AV's real-time scanner should still catch the file once it's no longer hidden inside of an archive. So the benefits of drilling down more than just a couple of levels might be minimal.

Is there any significance to all this? IMO, yes and no. Yes, in that we all would like for an AV to stop malware the first time it encounters a bad file. But also no because I'm protected anyway. Why waste my time or my computer's resources?