I feel I can speak with some authority on this one, having posted a number of times on photos from this position and also discussed the matter with various screeners.
The key here is to ignore the slopes that we know are present at this spot and level it according to the 'aesthetics' argument - i.e. so it looks level.
I have chosen one of my photos to illustrate the problem you have here due to the amount of zoom used:
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Photo © Paul Markman
It shows the same piece of background, but at a wider angle. I also used the grass horizon to level this photo, but you can see in the wider angle shot that the ground is not level - i.e. it rises from the left edge of my photo, then drops again to the right edge. Hence there's a bit of a hump in the middle (underneath the belly of my 757). Thus I was able to level the photo using the right and left ends of the grass horizon, ignoring that little hump in the middle. You have the 'hump' towards the right, so effectively are using a horizon that will be slightly sloped. Make that look level, as you have done, and everything else will be 'pulled' slightly in a CW
direction - hence the apparent need for CCW rotation.
I hope I am getting across what I am trying to say. In essence, if that grass horizon extended to the right of your crop, it would drop down and, if you then used the 'new' right end of it, I think the verticals in the left of the shot would look okay.
Put all this in a comment to the screener
- that grass slope is misleading you and the background verticals should be vertical - especially that one just to the left of the engine cowling. You can use my photo if you like to illustrate that little hump in this area.
There - too long a post once again! I just can't seem to help myself