Why a mirror image is so hard to reject
Hold your left hand up next to your right. Fingers match, proportions match, nothing out of place. Now try laying one flat on the other, palms down, both pointing the same way, and watch it refuse. You could sit there turning them for an hour and get nowhere, because the two of them are reflections rather than copies.
Shapes with that property are called chiral, after the Greek word for hand. Chirality is the reason a rotation test bites, and it exposes something odd about your eyes.
Your eyes are built to ignore the difference
Show the visual system an object and its mirror image and it files the two as one object. That is a design choice rather than a flaw, and for almost everything you have ever needed to recognise it is the right call. A dog facing left is the dog facing right. A chair from either side is a chair. A face stays a face. Filing the left-facing and right-facing versions apart would cost twice the labour for half the knowledge.
Your brain generalises across reflections and never asks permission. Psychologists call it mirror invariance. You can watch it in a small child who reads a picture book held up to a mirror without complaint, then signs her name backwards and sees nothing wrong with it.
You never grew out of mirror invariance. Teachers taught you to hold it back, and only where letters are concerned.
Then writing broke the rule
The Latin alphabet is stuffed with mirror pairs that mean different things. b and d are one shape and its reflection. So are p and q. Nothing else you look at all day behaves like that.
Stanislas Dehaene argues that reading hijacks a patch of visual cortex built to recognise objects, and that becoming literate means training that patch to stop generalising across reflections, for one narrow class of squiggle and nothing else. So a beginner who reverses letters is doing something ordinary. It is not a symptom of anything. The visual system is handling those symbols the sensible way, and the people who invented them never thought to consult it.
Where a writing system carries no mirror pairs, the error never appears. The alphabet is at fault here, not the child.
Which is why the test is hard
Put a chiral figure in front of someone and ask whether the second one turned or flipped. You are asking for the one distinction their visual system spent a lifetime learning to bin.
That override is what our mental rotation test measures. Your eyesight is beside the point, and so is whether you can make out the figure at all. The question is whether you can push past an old and useful default and keep hold of a difference the rest of your life trains you to drop.
The way people fail gives it away too. They seldom pick something absurd. They pick the reflection, and they pick it with confidence, 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. Fall for a mirror image and your object recognition is doing its job. I would be more curious about the person who breezes through, whose visual system is up to something unusual.
That you can override defaults, at a cost. The override takes effort and it eats time you can put a stopwatch on, which is the whole point of the timing result behind the task. Work that fights an automatic process is slower than work that rides along with it. Every time.
That the strategy matters more than the effort. Turn the whole figure and reflections cost you nothing worth mentioning, because a reflection never lines up. Hunt for a landmark and you walk into the trap, since landmarks survive a flip intact. Same person, same ability, a very different score depending on the route they took.
Where it stops mattering
Chirality reaches a long way past puzzle books. It runs through chemistry and biology, where two molecules that mirror each other can behave in completely different ways inside your body, which is how "the same compound" turns out to be two rather different things. Same geometry, far more at stake.
Your score on a fourteen-figure test carries a good deal less weight. It tells you which strategy you fall back on and roughly how fast you can run it, and it will improve if you sit the thing a few times. Have a go, and watch whether you are turning the shape or hunting for the dot.