I think I've figured out why. This system doesn't analyze how good a picture is but for how long you will remember an image.
If I take a landscape with low lying mountains on the horizon, the brain can easily confuse it with another picture of a low lying mountain on the horizon. While the picture might be very good and well printed, it's not memorable per say.
On the other hand, take a nice portrait. We have brains that are designed to remember faces. If you show me a picture of somebody with a big nose and then another picture of somebody with a small one, anybody (except those with face recognition blindness) will be able to tell the difference.
That's why portraits score higher on the engine.
As for stuff that is out of the ordinary and more graphical, it's easier to remember....
It all has to do with memory...
And if you manage to get a picture with good composition and a high memorability score, you have a definite winner.