I think it might be how they balanced convincing Lords to join your Kingdom in a fairly recent batch of hotfixes. Whether I was a vassal or a king, I always would have to hunt down the rival clan leader to recruit them, spend an unreasonable amount of resources after passing the Charisma checks, or capture all their holdings and wipe their empire to try and recruit them on the cheap. Meanwhile, lords leapfrog between all the other remaining kingdoms after their kingdom gets wiped, but I always have to hunt down the clan leader to recruit never just get a clan like "Oh hey cool if I join you guys?", but Monchug Khan is out here just randomly scooping up Sturgian and Imperial lords and 6 Merc factions rollin hella deep.
TL;DR: Player needs to jump through a million hoops to get lords and mercs with limit caps, but other Kingdoms just magically acquire them with no merc limits.