This has been interesting re: the visual outward appearance versus reality of a “hollow back”. I think if you consider the structure of the horse’s spinal column vs the spinal processes that contribute to its outward appearance or topline, you may understand further why it is more useful to look at the whole horse, and the placement and reach of hind legs under the body rather than the slope behind the saddle.
The spinal column that houses the spinal chord is itself is fairly rigid and straight. The horse does not have the pronounced S curves a human spine has. Deformities or individual differences in the shape of the vertical spinous processes that rise out of the spinal column can create a variety of “looks” to the top line - flat, curvy etc.
Of course, the length of this straight spine can be higher at one end than the other - like a pole being held perfectly horizontally, versus a pole (like a mop or broom handle) held with one end higher. “Lowering the haunches” will look different in different horses depending on whether their spine is a naturally uphill pole, level pole or down tilted pole. But the far end of the pole can still lower relative to the natural placement of the front end of the pole.
In fact, in light of these facts, my opinion is that “hollow back” is a misnomer and when we say “hollow back” we are actually speaking of a horse with braced back muscles, inverted neck, and unengaged hind end. Not a back that intermittently sags in the middle because the horse is being ridden improperly or has a bad moment.
Look at this image of a horse’s spine and see how the withers formed by long tall spinous processes - not the shape of the “spine.” Imagine how differences or irregularities in how these processes might develop could change the outward appearance of a back.
https://www.irinfo.org/articleofmonth/images/4_1_2011_robson_4.gif
Check out this article from Equus regarding sway backs. We are not talking sway backs here, but as it also has good information on general back anatomy.
https://equusmagazine.com/management/swayback-in-horses-8221
Read the subsection titled "Spinal Designs and Misalignments"
“Appearances are deceiving even with normal horse backbones. The familiar rise and fall and rise again of the equine topline does not describe the path followed by the spine itself. Instead, the vertebrae from chest to tailhead follow one another in a nearly horizontal straight line. The undulations on the exterior of the horse are created by finlike projections, called dorsal spinal processes, on the vertebrae’s upper surfaces…”