When you are in forward flight, the helicopter doesn't require as much pitch because the rotor doesn't have to "work" so hard due to the calm air constantly passing through and under the blades. When you get into a hover, the air underneath the rotors gets very turbulent, and the rotor has to suck additional air into the rotor from the top. It requires a lot more power and pitch to hover.
Part of this is also due to the relative speed of the air across the blades. When you hover the blade speed is exclusively dependant on the rpm of the rotor. When you are in forward flight you get to add the forward speed to the head speed which increases air flow. There is a bit of a loss on the blade headed down wind, but there is still an apreciable difference between a hover and forward flight. This is why you need to reduce throttle when you fly forward and advance the throttle when you hover.
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