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 over 1 kg = +59 degrees on the
hydrometer (with Oechslescale)

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 20 grammes under 1kg, i.e., minus 20 hydrometer
(with Oechslescale) degrees (coloured field)