Basic facts:

Volume of 1 kg dissolved sugar is 0.63 litres

Weight of 1 litre of dissolved sugar is 1.59 kg

Weight of 1 litre of water is 1 kg (1 000 grammes)

Weight of 1 litre alcohol is 0.8 kg (800 grammes)

100 ml of dissolved sugar = 159 grammes (100:0.63)

900 ml water = 900 grammes

159
900
1059 grammes

This is 1059 grammes = 1059 degrees on the hydrometer

When the sugar has fermented to alcohol it looks like this:

100 ml alcohol x 0.8 = 80 grammes
900 ml water = 900 grammes

80
900
980 grammes.

This is 980 hydrometer degrees