First, why do you assume they were looking through the viewfinder?  For all
you know, the person taking the picture didn't even know what a viewfinder
was and just pointed the camera in the right direction and pushed the
button.

But, to really answer your question, composing a picture in a viewfinder
really is an abstract skill.  You look into the window and the stuff you're
looking at certainly looks straight.  To visualize it as the camera sees
it, you need to mentally transpose the scene into another coordinate
system.  The camera is tilted to the right, the image will be tilted to the
left.  It's not as obvious as it may seem.

One of the difficult things about teaching a skill to other people is
remembering what it was like when you first tried to learn that skill.

Glasses sometimes make it hard to see the whole viewfinder. A lot of
people don't understand the meaning of those edge of picture markers
you see in rangefinders and point and shoots; they think that if it is
in the viewfinder it will be in the picture. People don't see the
distortion caused by the lens; their brains automatically correct for
it and they don't see it until the picture is printed.

People have a problem with paying attention to everything that does
appear in a viewfinder. They see the subject, but not the tree growing
out of his ear. They see the view, but not the power lines in front of
it. They see the landmark, but not the pile of unsightly trash around
the dustbin in front of it.

One reason is that we often see what we EXPECT to see.  This is a
characteristic of human vision.  The brain does not receive a
"photographic" like rendition of the external scene.  There is a
fantastic amount of data compression in the human visual system.  If
we think we are looking at a familiar object, we will see the
"object", not the pixels that make up the object.  This is the origin
of so many optical illusions.

One result is that we do not see what we do not expect to see when we
look through the viewfinder. I myself have shot oh so many bad shots
because my brain ignored distracting objects that obviously must have
been in the viewfinder, but they were not what my brain was expecting,
so it tuned them out. Composing through a camera viewfinder is a
learned art.

In one of the examples given, everyone knows the horizon is level, so
it "sees" the image that way.

Now, unfortunately, because of some quirk of the vision system, it
does NOT do the same processing when viewing a print as it does with
the original scene, even when viewed through the viewfinder.  This is
weird, in my mind, but true.  Best example is when we see the scene
that has a non-normal color temperature.  We do not notice that
coloration, but boy when we see a print, even a colorimetrically
accurate print, we really notice the color :-(

What will you do with this information?

LOL, it's true!

I have given my camera to people to take a group photo with me in it (not by
choice, as I prefer to be behind the camera) and all but one occasion they
have not turned out very good.  Prime lens, so no zooming to do, told them
where to stand, I set the camera up so all they had to do was stand still,
put the heads in a certain position and press the button, fool proof (or so
I thought)!   The one time it turned out well was with a total stranger.  So
if you're that person, thanks.

On one occasion, I forgot to take the continuous shoot off and I ended up
with a burst of 5 photos, with a group of four all in the bottom right hand
1/4 of the photo and of course nicely focused on the background!   I can't
believe how they got it so wrong.  It was on a wideish lens and they must
have used their own foot zoom when I went to join the photo.  Who would
frame a group photo in the bottom left hand quarter?!?  I didn't have the
heart to say anything, just deleted it afterwards.