So... I have set up my aircraft with an empty weight of 1518.1 lbs and a CoG(Lon) of 38.56, added all the corresponding info into the aircraft configs like so...

35.00,1518.1  35.00,1950.0  39.50,2400.0  47.30,2400.0  47.30,1518.1  to create the polygraph.

Returning to the flight planning page and entering zero for all the weights why does it show an empty weight of 1505.4lbs and a CoG of 38.48?

Edit to say...This is the PC version WIn7

I recall that its to do with taxi fuel.
When you taxi out your gross wt reduces by the taxi allowance to give takeoff wt rather than ramp wt. The assumption by SD is that you don't try to go flying with no fuel!