Just checked my Back to the Future boxset. It's an early release region 4, that didnt get the extra commentary track on the later released Region 1. Also has some framing errors.

From Michael D's Region 4 DVD Info Page:

Yes, to repeast that the Region 4 release has NO commentaries. What it does have is an interview/Q&A session with a live audience that doesn't even run the length of the feature.
The Region 1 get's an interview for ALL 3 films, plus commentaries for all 3 films. The 3rd "commentary" you refer to is no doubt, the awesome trivia track.
Yeah, the Region 4 version sucks big time, but we got it first. And I like Black more than blue for my case.

So, it's gonna have to go for a new Region 1 version if it has a commentary track as good as you say.

Yes, agree with you on the Spinal Tap track ThatArtGuy. That just kept the joke going.