Static Control of LDO built with Folded Flipped Voltage Follower

Hello,

I don't understand the static control of this LDO which is realized with 
a Folded Flipped Voltage Follower.

The current of current source MB1 must be higher than the current of 
current source of MB2 so that VOUT can be controlled to the desired 
voltage VREF. I know this because I have run the simulation but I don't 
really understand why. Why is VOUT=VREF (if the control circuit works 
fine)?

I would be glad for any explanations, thank you!

BR