I've done a bit of digging...
The later fuel sender has highest resistance when the fuel tank is empty. This is the better system because it means that, should the connection between the sender and the fuel gauge in the cluster fail, the gauge will immediately show empty. In other words, any increase in resistance will reduce the perceived amount of fuel in the tank.
The early sender works the other way around. When the sender's electrical resistance is at its highest, the fuel tank is full. This system is a bit daft because any electrical fault that results in greater resistance, however minor, will cause the gauge to show more fuel in the tank than is actually the case. I'm not surprised that Audi changed this. What surprises me is that, after however many decades of making cars, they hadn't realised the flaw in this system a long time before the A2 was ever conceived!
Anyway, if we assume that your car is using the later sender, the symptoms are easily explained by a dodgy/corroded connection. More resistance means less fuel. So, when your fuel tank is full, the gauge shows 3/4 full due to the added electrical resistance in the circuit.
It does not, however, explain why the fuel gauge stops falling when it gets to the halfway point, as you explain in your original post.
Cheers,
Tom