As another comment said, the wave behavior when not measured is hard to explain if one thinks of photons as little particles that classically would need to go through one slit or the other. It seems each one goes through both slits and self-interferes.
And when measured, sure enough they act like little particles that need to go through one slit or the other.
So what’s surprising about the experiment?
Of course, when you interact with things they change
As another comment said, the wave behavior when not measured is hard to explain if one thinks of photons as little particles that classically would need to go through one slit or the other. It seems each one goes through both slits and self-interferes.
And when measured, sure enough they act like little particles that need to go through one slit or the other.