The 7 litres is included in the 63 litre capacity.
The actual capacity would be slightly more, as the 63 litres does not necessarily included the litre or two in the fuel lines and the fuel filler neck if you brim the tank. But not 70 litres. Maybe about 65 or 66.
The light is *supposed* to come on when only 7 litres are left, so you should be able to get about 56 litres into the tank.
So you've got 13 litres "missing", not 20 litres.
That may well just be inaccuracy in the gauge - maybe you were driving up an incline and the fuel sloshed away from the float for long enough.
In my 156, the light often comes on, but when driving the car again later, the light will be off again.