Why does the static pressure inside the cockpit vary from outside static pressure?
You're referring to this statement in the Pilots Handbook of Aeronautical Knowledge, Page 7-12: 


  Some aircraft are equipped with an alternate static source in the
  flight deck. In the case of a blocked static source, opening the
  alternate static source introduces static pressure from the flight
  deck back into the system. Flight deck static pressure is lower than
  outside static pressure.


The reason for this error with alternate static source is that the air flowing around the airframe creates a slight vacuum inside the airframe, thereby lowering the barometric pressure and generating erroneous readings in the pitot-static system.