synaptics only recognises the SoftButtonAreas option if the option ClickPad was set _before_ the button options. Otherwise, though the property gets set on the device, it isn't handled by the driver and has no effect.
http://patchwork.freedesktop.org/patch/12134/
commit 86460318ba5606ef41712b52e50bf7b2ba083226 Author: Peter Hutterer <peter.hutterer@who-t.net> Date: Thu Aug 23 16:22:31 2012 +1000 Recognise the soft button area property before clickpad is enabled
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