Sorry my poor English. This circuit is made by the n-MOSFET AO3400. Right MOSFET is normally connected MOSFET, and left MOSFET is reverse connected MOSFET. And right graph shows the relation between Vgs and Vds simulated by LTSpice. The blue curve is normally connected, and orange one is reverse connected.

At low Vgs, reverse connected MOSFET is conducting current by body diode, and starts to drop to GND level at 0.5V. Why reverse connected MOSFET starts to turn on at low Vgs?