Ok I mixed up some of each and measured the resulting volume, the missing factor is how efficiently the sugar dissolves. 120ml of water and 120ml of sugar yields about 180ml of 1:1 syrup, and 60ml of water and 120ml of sugar yields about 120ml of 2:1 syrup, so the ratio is in fact 3:2.

Your chart is correct, the only real adjustment is that 2:1 syrup is 1.5 times as sweet as 1:1 syrup.