It appears the regulator is going open loop. What's the maximum voltage it can output?
You have an emitter (BJT) source (MOSFET) follower. The voltage on R1/R2 will always be lower, than the base/gate voltage. The transistor starts to turn on, when a certain threshold between the base and emitter/gate and source is exceeded. The voltage on R1/R2 rises, thus reducing the potential difference between the base-emitter/gate-source and turning the transistor off. The circuit settles with the voltage on the emitter/source a bit lower than the voltage on the base/gate.
BJTs have a turn-on threshold of about 0.7V (double that to 1.4V for a Darlington pair such as the MJ11028G) and MOSFETs a couple of volts, hence why the voltage across R2 is lower, than that across R1.