Stitching module's strange behavior (Part I)

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I came across a very strange behavior of the Stitching module.
Please read the whole question to understand the details.

I'm tried to stitch two images named "_pano1.jpg" and "4.jpg" with "match_conf = 0.3".

Here's the debugging output information:

Features in image #1: 853
Features in image #2: 370
Finding features, time: 0.696868 sec
Pairwise matching
1->2 matches: 44
1->2 & 2->1 matches: 67
.Pairwise matching, time: 0.0538355 sec
Removed some images, because can't match them or there are too similar images: (2).
Try to decrease --match_conf value and/or check if you're stitching duplicates.
Need more images

Then I tried to change the value to "match_conf = 0.1". Here's the output after that:

Finding features...
Features in image #1: 853
Features in image #2: 370
Finding features, time: 0.716048 sec
Pairwise matching
1->2 matches: 284
1->2 & 2->1 matches: 384
.Pairwise matching, time: 0.104792 sec
Removed some images, because can't match them or there are too similar images: (2).
Try to decrease --match_conf value and/or check if you're stitching duplicates.
Need more images

At the same time when I'm trying to stitch 3.jpg and 4.jpg with "match_conf = 0.3" the output looks like this:

Finding features...
Features in image #1: 441
Features in image #2: 474
Finding features, time: 0.697173 sec
Pairwise matching
1->2 matches: 62
1->2 & 2->1 matches: 90

So the module stitches them successfully, but if I try to change the value to "match_conf = 0.1", then it tells me that it can't stitch 3.jpg and 4.jpg:

Finding features...
Features in image #1: 441
Features in image #2: 474
Finding features, time: 0.780463 sec
Pairwise matching
1->2 matches: 198
1->2 & 2->1 matches: 335
.Pairwise matching, time: 0.103776 sec
Removed some images, because can't match them or there are too similar images: (2).
Try to decrease --match_conf value and/or check if you're stitching duplicates.
Need more images

~ Why _pano1.jpg and 4.jpg don't stitch no matter what's the value of "match_conf" even though 3.jpg and _pano1.jpg basically contain the same information for stitching with 4.jpg?

~ Why it tells me that it can't stitch 3.jpg and 4.jpg after lowering the "match_conf" value?