My understanding is that a lot of the weirdness is that there may be no such thing as a photon, that is, what we call a photon is the em field interacting with matter(electrons really), in transit there is no such thing as a "single photon" That is, the em field is not quantized( or at least not quantized at the level of a photon) It is very possible to have your em field at "sub photon level" and the electrons at the far end will accumulate energy until a photon level is reached and presto a single photon experiment. What it is actually measuring I am not sure.

https://www.youtube.com/watch?v=SDtAh9IwG-I (Huygens Optics -How big is a visible photon?)