In an essay wondering why no one shot at the Trump assassin before he started shooting, a Philip Kraske brings up a good point:
Who would think a guy who got so close, so blatantly visible with a rifle, actually was a real assassin???
That's flatly not supposed to be possible, certainly not with SS SOP. Also a bad location compared to the taller buildings further back which had concealment. What if he's a good guy, in a situation where it's pretty clear communications were (at best) fouled up??
So the unbelievably could have worked in the assassin's favor. In particular for the two police snipers on the northern roof, who went from watching, to shock, and then action after the shooting started and are said to not be the ones who nailed him.
Especially since the ... NYT I think is credibly claiming, from their viewpoint the guy was in a reverse slope, might not have been seen until he raised his head etc. over the top of the roof.
https://www.unz.com/article/why-didnt-the-counter-snipers-shoot/