Backbone? Splay? Density?
Essentially yes and no. Backbone and splay can be controlled via socket width and depth. The glue bump needs to be taken into consideration when setting a knot, as this will also impact how the brush performs. It's hard to break it down into a single sentence, which is why Ken merely said feels different.
For example, if you have a 24mm knot, with 65mm loft, and 50mm free loft. The width at the top of the glue bump will most likely be 1-3 mm larger than the plug. So you will end up with a 26mm socket width. At that point, the handle isn't really doing anything much (will be controlling outer hairs a little, but nothing for the centre) to contribute to the feel of the brush, as it's all being done by the glue bump. Now you set it 1mm deeper at 16mm, with a 49mm loft, all of a sudden the handle is choking the knot and giving added backbone. This is why at Paladin they have the setup below. They can effectively set a knot in a socket and play with the loft to see how it feels.
Now if you set a knot with too low of a loft, it will have different characteristics, most importantly splay and flow. With splay, if you don't have soft tips, or the hairs are particularly thick (some 2-band), then it can lead to scratch/scritch, as well as throwing lather around the room. This then talks into flow, with the brush not releasing lather, and having to do exaggerated painting motions to release (you'll also see lather at the bottom of the knot around the handle). Density is kind of a separate thing, as that will affect how wide the glue bump spreads above the plug, and in that, is taken into consideration when setting loft for splay and flow.
This is where you pay the extra money to some of the artisans, as they just do their magic finding that 1-2mm loft/width window that will make the brush a pleasure.