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!

Spot on, thanks Pete.

As you quite rightly say, It seems that Skydemon takes into account the taxi time (fuel used) and deducts this to calculate take-off weight.

Yep, on a commercial aircraft load sheet (W&B) the fuel on board is reduced by the taxi fuel(which can be considerable with larger aircraft) so that take off wt is correct.
SD also does this, but the way it is implemented means that you get the result you found with zero fuel, put your taxi fuel into SD fuel window and you should get your empty wt that you entered.
regards
Pete