To attempt a disarm, Jorran must hit 15+2+4 = 21, AND he generates an attack of opportunity from Kal.
If he manages to hit 21, then there is an opposed attack check for the disarm, Jorran has a +4 to this check for his 2 handed grip, if he wins it, he disarms Kal and his lightsaber falls to the ground.
If he loses the check, then Kal can attempt ANOTHER check, Jorran still having +4 bonus, to disarm Jorran. If Kal wins, Jorran is disarmed. If Kal loses, the attack is over.
So, what I think happened is:
JORRAN TURN: DISARM
Jorran: Attack against 21 (kal's lightsaber) with 1d20+6, succed
Kal: Attack of opportunity against 16 (jorran's defense) with 1d20+5, fail
Jorran: Opposed disarm check 1d20+10 against 1d20+5, fail
Kal: Opposed disarm check 1d20+5 against 1d20+10, fail
KAL´S TURN