Cron,
A vector diagram is needed to explain it properly and my explanation is simplifying things slightly; but for a starter,
Imagine a fixed wing moving forward through still air. The angle of attack in this case is simple to visualise, it's the angle between the relative airflow (from straight ahead) and the chord line / rigging angle of the wing.
I think you will agree that this wing will deflect air downwards during its passage through the air (downwash).
Now imagine a second wing immediately behind and below the first. This wing will experience a different relative airflow because the air through which it passes has been deflected downwards. The angle of attack will be less.
Helicopter rotor blades suffer from the same thing as they all follow each other round the mast. The induced flow is the cause of the reduced A of A.
ShyT