Much easier to answer it in these simple terms in my view:
Power controls rate of descent. Pitch attitude controls speed. So you've set the pitch attitude for 70kts. Your normal power setting of 1700rpm gives you a rate of descent that'll take you to the runway in zero or light wind. But in a strong headwind, your ground speed is reduced, so 1700 rpm will result in you reaching the ground short of the runway. You can see that by looking forwards and seeing the runway numbers going up the windscreen. So you need to increase the power.
NS