Well let me tell you right off the bat. That a lot of the stuff being sold out there in my opinion is just not up to the job.
Not only, but very often people are searching for a good quality ceiling fix curtain track. That will also fit in to a bay window. The kind of bay window with a lower ceiling than the main part of the room. Just like in the image here.
Another big reason people want to use a ceiling fix track. Is because there is little, or no space over the top of the window. Simply no, or not enough wall to secure the curtain track brackets. So they have no choice but to fix upwards in to the ceiling.