You have a 'complicated' facial geometry in the sense there are some distance relationships that can't be changed but are a significant number of deviations from the norm. For example the S-N line distance is very short compared to the norm. So, they are kind of limited in how much they can 'balance' so that all is within the norms. Also, the teeth are kind of 'bucked out' which will limit the extent of CCW they can do because the CCW (posterior down graft) would tend to buck them out more as the teeth also get rotated in that process.
With reference to a horizont;
your occlusal plane angle is about 16 degrees
your Mandibular plane angle is approx 36 degrees
your OP is about 4 degrees in excess of the norm range of about 12 deg
your MP is about is about 9 degrees in excess of the norm range of about 27 deg
Sure, you could say "steep" but when ever you can MEASURE something relative to a line that could be used in a ceph analysis, you can compare HOW steep with reference to which line one is using to take the angle and then look up the norms.
Here, I'm using a 'true' horizont because your Frankfort line is pretty close to a horizont (a few degrees off but close enough).
I could also use the Steiner S-N line to measure the OP and MP. Your S-N line is about 8 degrees away from a horizont. With that system:
your OP is about 24 deg
your MP is about 44 deg
your OP is about 10 deg in excess of a 14 deg mean in Steiner
your MP is about 12 deg in excess of a 32 deg mean in Steiner
So you could say BOTH are 'steep'. But you can't really conclude that CCW to the maxilla (hence double jaw) is the 'answer'. If you think it is, calculate the amount of OP rotation that would be needed to offset (make less) the excess angle of inclination of the MP. Or just think intuitively in terms of knowing that CCW to the maxilla (here posterior downgraft) will be proportional to the rotation of the mandible. So, to make it easier, let's say rotate maxilla 4 degrees to get about a 4 degree auto-rotation of mandible. Make it 5, 6, 7...and still you won't be enough to offset the excess of the MP. But wait...there's an easier way to think about this. NOTE the PURPLE LINE I drew on your ceph. That's basically the line that would be rotated CCW. Note that the more that's rotated, the more your teeth are going to be 'bucked out' closer to a horizontal axis. In essence, you have some angle relationships that LIMIT how much benefit you would get from CCW.
Now your maxfax probably saw that kind of relationship and hence just suggested single jaw only. They could also look at the angle relationship between the MP and OP which are BOTH 'steep'. It's about 20 deg which is about within the norm for an OP-MP angle even though both are steep.
What they are looking at is DATA that would justify BSSO only. They can also see that although your S-N line is short and with a number of deviations from the norm length (something that can't be changed), your SNA is within the norm (that refers to maxilla angle relationship) and it's only your SNB (that refers to the mandible) that is LESS than norm for that angle and hence can be isolated for single jaw advancement.
So, I've found what they most likely would be looking at and how they would look at some of these relationships.
Now, it isn't easy for me to EXPLAIN this stuff. Like I never know if people getting this stuff are even 'in tune' with some fundamental geometric relationships which I'm using to explain things. If not, not my task to fill in too many gaps.
So, here, I'm just conveying the most likely HOW and WHY the doc who suggested BSSO only is looking at it and conveying such in the event you would like to think more about the conclusion of: 'You need bimax CCW because you have a steep OP'. A steep OP is not the only thing a max fax looks at. If that were the case, there would not be all those lines and angle measure relationships made from them which you can see on the ceph and measures of them on the ceph analysis.
