Why a mirror image is so hard to reject
Hold your left hand up next to your right. Same fingers, same proportions, same everything. Now try to lay one exactly on top of the other, palm down, both pointing the same way. You cannot. No amount of turning will do it, because they are reflections rather than copies.
Objects like that are called chiral, from the Greek for hand. They are the reason a rotation test is hard, and they expose something genuinely odd about how you see.
Your eyes are built to ignore the difference
The visual system treats an object and its mirror image as the same object. This is not a defect. For essentially everything you have ever needed to recognise, it is exactly right: a dog facing left is the same dog facing right, a chair seen from either side is a chair, a face is a face. A system that filed left-facing and right-facing versions separately would be doing twice the work to learn half as much.
So your brain generalises across reflections automatically, without asking. Psychologists call it mirror invariance, and you can see it working in small children, who will happily read a picture book held up to a mirror and write their name backwards without noticing.
Mirror invariance is not a bug you grew out of. It is a feature you had to be taught to suppress, and only for letters.
Then writing broke the rule
The Latin alphabet contains mirror pairs that mean different things. b and d are the same shape reflected. So are p and q. Nothing else you look at behaves like this.
Stanislas Dehaene's account is that reading repurposes a patch of visual cortex that evolved for recognising objects, and that part of becoming literate is training that patch to stop generalising across reflections, for this one narrow category of squiggle. Which is why reversing letters is completely ordinary in early readers. It is not a symptom of anything. It is the visual system doing the sensible thing with a set of symbols that were designed without asking it.
Writing systems that avoid mirror pairs do not produce the error. The problem is in the alphabet, not the child.
Which is why the test is hard
Now put a chiral figure in front of someone and ask whether the second one is a turn or a flip. You are asking them to make precisely the distinction their visual system spent a lifetime learning to throw away.
That is what our mental rotation test is actually measuring. Not whether you can see the figure, and not how good your eyesight is. Whether you can override a very old, very useful default and hold on to a difference that nearly everything else in your life tells you to ignore.
It also explains the specific way people fail it. They rarely pick something wildly wrong. They pick the reflection, confidently, because at the level their visual system reports upward, the reflection is the same object.
What it shows about your mind
Three things, and none of them is "how clever you are".
That you have a default, and it is usually right. Being fooled by a mirror image means your object recognition is working normally. Someone who found this trivially easy would be someone whose visual system was doing something unusual.
That you can override defaults, at a cost. The overriding is effortful and it takes measurable time, which is the whole point of the timing result behind the task. Cognitive work that runs against an automatic process is slower than work that runs with it, every time.
That the strategy matters more than the effort. People who turn the whole figure reject reflections almost for free, because a reflection simply never lines up. People who check for a landmark instead get caught, because landmarks survive a flip perfectly well. Same person, same ability, very different score depending on which route they take.
Where it stops mattering
Chirality is not just a puzzle-book curiosity. It runs through chemistry and biology, where two molecules that are mirror images can behave completely differently in the body, and it is why "the same compound" can be two quite different things. That is a much more consequential version of the same geometry.
Your score on a fourteen-figure test is a smaller matter. It tells you which strategy you default to and roughly how fast you can run it, and it will improve if you take it a few times. Have a go, and notice whether you are turning the shape or hunting for the dot.