I've kinda noticed this too. Mind you I'm currently playing as the Cumans in high and decided to try and hold Lesser Kazar so it was pretty inevitable that the Horde was going after me!!

Still after killing two Khans in three sucessive battles they decended into Rebels - must have killed all the princes too. Having only vanilla spears available was a bit of a pain I can tell you!! Still, Cuman Warriors, Bashkorts and Steppe Heavy cavalry plus a couple of merc. Pav. Arbalesters and a unit each of Arquebusiers, Billmen and Halbardiers who were worth their weight in Florins proved more than enough to yield 2:1 and 3:1 victories.