1 speed and 2c are both pretty significant, though 13 difficulty and 15 difficulty are basically the same thing barring some extreme builds.
I'd say the ability to use Svaerdstav with a shield and one handed lance thrust animation on horseback pushes it over LHB for flexibility, I rarely ever see anyone doing guandao cavalry because of the terrible penalties it has. For no shield infantry, Svaerdstav is kind of lackluster vs. stuff like LHB/Scottish Halberd/Glaive or the 2D weapons.
The reason why Svaerdstav gets high bonuses is it uses imodbits_spear or something, most heirloom bonuses are based on rounded percentage additions based on an imodbits value. Some weapons get comparable or better bonuses, especially if you account for +3b/p being a bit better than +4c (See any staff, for example).