Basic facts:

1 litre of water weighs 1 kg (1 000 grammes)

1 litre of alcohol weighs 0.8 kg (800 grammes)

1 litre of water weighs 1 000 grammes = which is neither more or less than 1 kg. That gives 1000 degrees on the hydrometer (specific gravity).

1 litre of alcohol weighs 800 grammes. This would be 800 degrees on the hydrometer (specific gravity) if it had a scale long enough.

100 ml alcohol x weight 0.8 = 80 grammes

900 ml water = 900 grammes

80
900
980 grammes

980 grammes is (specific gravity) 980 hydrometer degrees