Sloped Zero Fuel Line on C of G chart
I have recently being doing a bit of flying on a Piper Seneca 2 with a VG kit. The stock airplane has a 4000 lb zero fuel limit which is a flat line from most forward C of G to most aft C of G. The VG kit comes with a W & B supplement. The zero fuel limit is now 4040 lbs at the most forward C of G to 4163 lbs at the most aft C of G. I am trying to wrap my head around how;
1) A VG kit can affect a structural limit, and
2) Why the zero fuel weight would change with C of G location
Anyone got an explanation ?