Sometimes questions can be answered by plugging them into a little math.
Divide 170 into 180 or 180 into 170 and you will reveal that the weight discrepancy is about 5 1/2 %.
A typical bullet alloy for handgun bullets might be 20 to 1 lead to tin,or roughly 5% Tin.
So,if the tin were absolutely weightless,which it is not,the 5% tin would not cause a 5 1/2 % weight shortage.
I suspect error in calculating the initial volume of the mold...such as not allowing for the shrinkage,or the lube grooves,etc at the drawing board.
It also might not hurt to verify your scale with either test weights,(best) or samples of quality jacketed bullets.
Of course,test weights are best,but not always available.
Generally bullets from Sierra or Nosler or Hornady or Speer,etc,will be very close to advertised weight. Close enough to confirm the scale is pretty good or grossly off. I don't mean 1/10th grain accurate,or even 1 grain accurate,but they will suggest whether a 10 grain discrepancy is the scale or the bullet.