Because the Figure 1 you posted refers to a *4-terminal* device, not a 3-terminal one. If you look at the schematic symbol in Figure 1, you'll note that the body terminal is a separate terminal not connected to the source terminal. MOSFETs for sale are almost always 3-terminal devices where source and body are connected together.

If memory serves me right (not 100% sure -- seems to be corroborated by [this handout][1]), in a 4-terminal device there is no difference between source and drain, and it is the gate-body voltage that determines the on-state of the channel -- with the caveat that the body is supposed to be the most negative voltage in the circuit for an N-channel device, or most positive voltage in the circuit for a P-channel device.

Look carefully at the conventional schematic symbols for N- and P-channel MOSFETs ([from Wikipedia][2]):

![n-channel][3] ![p-channel][4]


and the Wikipedia figure on [MOSFET functioning][5], and you'll see the body-source connection.


  [1]: http://www.ece.nmsu.edu/vlsi/EE221-Sec5-1.pdf
  [2]: http://en.wikipedia.org/wiki/MOSFET#Circuit_symbols
  [3]: http://upload.wikimedia.org/wikipedia/commons/thumb/6/62/IGFET_N-Ch_Enh_Labelled.svg/80px-IGFET_N-Ch_Enh_Labelled.svg.png
  [4]: http://upload.wikimedia.org/wikipedia/commons/thumb/0/0c/IGFET_P-Ch_Enh_Labelled.svg/80px-IGFET_P-Ch_Enh_Labelled.svg.png
  [5]: http://en.wikipedia.org/wiki/File:MOSFET_functioning.svg