Matchbox detecting diffs where there are none

I am trying Matchbox so I’m not an expert and I might be missing something but Matchbox is finding diffs where there are none. I am using Video Files to match and feeding it the old and new reels, which are exactly the same in size, codec, length (except for the differences, of course).
MB is spotting differences in the Diff layer, yet not showing any red overlay on the center screen unless the the diff delta threshold is set to over 77%.
Is this expected behavior? Is there a way to change the threshold of the detection itself?

That’s an interesting situation.
Normally I’d say “there must be something different”, but it sounds like you’ve been careful about the codecs etc.

Are they definitely “visible diffs”? Matchbox creates several kinds of “diff” and visible comparison is just one of them.
Is there an overlay across the middle of one?
Have you accidentally got the cropping set to include timecode (or other) burn-ins?

I just loaded the same file on both sides and, as expected, no diffs were found. So that works.

I see the exact same picture on both sides, and no red overlay on the center screen, unless I turn the Visual Delta Threshold to over 75%. It’s finding diffs even before any changes have happened, so Timecode burnin is the same.
I think it might have to do with a slight change in the brightness of the quicktime (maybe a different LUT?) It’s not evident on most shots, but it is on some others. That might be triggering the diffs to show up. But they are so similar! I’m even getting diffs on the leader.