A Mystery to me, can you explain it?

[Andy Stringer]

I compared the two fence posts side by side, just the shaded sides that face the camera, with no changes to the original exposure. This removed any 'optical illusion' about the brightness of the posts when compared to the background. The difference in brightness is only about 2/3 of one stop. I'm sure this is just due to a slight difference in the ambient lighting. The posts are leaning in different directions so the sides facing the camera may not be catching the light in the same way, and no compensation was made for this.

There's no way that a camera exposure glitch could turn a bright background into a black one, yet still get the two posts within 2/3 of one stop, so I stick with my assertion that the answer to the OP's original question is that the background was too dark to register at 1/1600s.