You start off with ZA lets say
You must kill the final boss of ZA b4 your next random is ZG (100%)
then u must kill final boss of ZG for yr next dungeon to be ZA (100%)
its alternating dungeon in short, if u quit the party half way, you will keep getting ZG ZG ZG ZG ZG ZG
Disclaimer: I'm not the one to control the random, but this is what i tested for 20-30 times que-ing as tank and i saw the trend. Not once it missed this rotation.
In other words, you will never have random ZA ZA or ZG ZG if you killed their last boss.