Somebody may have added extra sound deadener. I believe it was a dealer option OR your paint has fallen off OR Luigi was having a very bad day and his already very rough painting process was even more hit and miss the day your car was built.My very original '74 GTV has no factory paint on the "sound deadener" There is likely paint under it.
But it has been proven many, many times, even by factory documents/photos that under the wheel wells should be body colour. Think about it; it would cost Alfa Romeo more to have painted the wheel wells black. Instead they painted the car and wheel wells, and squirted some paint (but not a lot) underneath and in the cabin area (my car had no paint on the bulkhead, inside of roof, etc.).
It is simply amazing that any of these cars have survived for so long, considering how poorly painted they were. A tribute to emotional owners.