I kind of knew all that, but at least we're now in common understanding.
sounds like you need a better surveyor then plot it on a section drawing and hey presto - a design that works.
Doing things "by eye" will get you so far, but where you bend further and go deeper, then you need to do it better.
sounds a little like someone is simply taking horizontal distance from one tie-in point to the other and ignoring the impact of the vertical leg?? Small deflections you'll get away with it, but I guess creeks are deeper and hence more impact on horizontal length
Kind of difficult to see without pictures or drawings. If it's consistently short, then add some green lengths to your sag design to compensate?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way