The Gleeful Grognard wrote:
"but the GM might adjust this number."
Covers it imo. Remember, bulk exists because too many people were handwaving encumberance and finding it a pain. So this is a way for it to still have an impact but be less stressful to track.
For a person carrying another character you are dragging them in most cases. Meaning you require both hands and move slower.
Keep in mind bulk is not weight, and if the creature is concious and holding its self up on the back of a creature in riding equipment, there is no way the creature is feeling the same level of difficulty in carrying it as if it were a limp corpse laden in gear.
So you wearing bulky stuff makes it harder for you to move about, but not necessarily the same level of difficulty for others to move you about.
Bulk is an extreme abstraction.
How do you get 12-13 bulk in equiment? That is a huge amount of large weapons. Remember that 1 bulk items are all treated as light for a horse. The 14B 1L I mentioned above included 6B for the person.
I would still wager paizo intends mount restrictions to be size, training/willingness and to not be bulk based though.
An average limp, unconscious medium person only weighs 30-60 pounds. It's very important to keep this in mind at character creation so you don't make your character heavier than they should be.
Most likely closer to 30. Unconscious people are hard carry, so it would have to be the lower end of the weight range per bulk.
The strongest man in Golarion can never carry more than 190 pounds (19 bulk), and then only if it's optimally shaped for carrying. It's likely that most large furniture is built in place and never moved, since a more average human probably would struggle to budge it.
If Bulk is a part of the game, then it's a part of the physics of the game, and we must consider its ramifications.
Much like how the lack of any mechanical effect due to age suggests that the elderly of Golarion are neither feeble nor wise, but instead...
To that last point, I don't think that's true at all. I think Paizo just got rid of the mechanical penalties because it was an unpopular holdover from earlier editions. A lot of people I know who play have always ignored those rules.
And I think the elderly still are.. well, less strong and robust as they were, when they were younger. It's just that they don't penalize the player characters for choosing to be hold, but the CRB even says "There aren’t any mechanical adjustments to your character for being particularly old, but you might want to take it into account when considering your starting ability scores and future advancement."
NPCs are probably still going to be less equipped for adventuring and things if they're elderly, just not player characters because they didn't want to have those penalties anymore.
Sometimes things in the game are made the way they are to make it less complicated for us as players, but in-world it's not actually how it works. Bulk is one such thing - a weapon might be 1 or 2 bulk, but when you're talking to a shopkeep or whatever, you would say "this is a 5 lbs. weapon". The actual numerical weight might be important to the story, but it's not systemically.
It's simply that the actual weight of things is not important in the sense of how we track our encumbrance, so they abstract it with bulk. I'd also think it's less bulk on certain mounts because they have more ways to distribute the weight than humanoids do, so it's less cumbersome for them, hence the lesser bulk.
I see it a little like the scale of what you're talking about matters - like how in Starfinder, a starship weapon might deal 1d8 damage, but if you were to shoot it at a human standing on a planet, it would completely destroy them with one shot. Similarly, what is 5 bulk to a human is not going to be 5 bulk to a dragon.