It's possible that horror isnt the mainstream and only fans go watch it, giving it a good rating because it's done well.

Pure speculation though, just wanted to provide a counter argument for fun.

"Outside of that the anomaly A- grades went to Five Nights at Freddies"

Seeing this makes me question it. My kid loved that movie so I've seen it a handful of times and it was better than I expected it to be, but still really bad. Getting a super rare A- shouldn't even be a question here. It could be a C tops.