Jihad: an unusual outcome

[Gilrandir]

It seems like my last Turkish campaign abounded in strange things. One more happened to my jihad.
It was directed against Rome. In the (autoresolved) battle I defeated the Pope. He was besieged in the castle and I was informed that the siege would last for 4 years. When 3 out of 4 years had passed I got a message that the Pope was killed (have you ever seen the leader of a faction killed before the siege was over?) and the faction was reduced to the rebel status, but the number of the besieged allowed the siege to last forever unless I storm the castle.
No matter how I tried I couldn't direct my jihad onto the castle to make it storm the latter. So I brought into the province some more armies hoping that the next year I would try to storm the castles with non-jihad troops (I concluded that jihads are not meant to storm fortresses - am I right?).
But here comes the strange thing (well, strange for me as I have never encountered it): after I hit the end of the turn, I got a message saying that the rebels agreed to relinquish the province without further fighting so my jihad was successfully completed.
Usually when you besiege a fortress you get all kinds of messages about the SIEGE, not about the possesion of the PROVINCE. How can a besieged garrison leave a province without leaving the castle first? Can this happen to a usual siege or it is limited to jihads (crusades??) only? Perhaps someone had something like that or has a plausible explanation?

[I of the Storm]

This peaceful handover of provinces is limited to crusades (and obviously jihads too, though I only encountered it with crusades). Rebels do that comparatively often, factions only do it sometimes but only when you are allied to them.