Nice spotted, but 22° is just my mistake in the drawing for the top rails (I tried several different angles before settling with 22.5). The error is too small to have an impact in reality since it’s hard to make bevel cuts with more than about 0,5° precision.
I’ve solved the problem for this build through help from Compound Miter Saw Calculator which states 15.7° (a lot less compared to 20.9° from SketchUp), but I was hoping to be able to trust SketchUp for planning any future and more complicated projects and compound angles. Therefore I’d really like to get to the bottom of this and understand where things go wrong.