Most props use a cambered airfoil. Cambered airfoils produce zero lift at a small negative angle of attack. The "pitch of most props is measured to the flat of the back of the blade for convenience. Using this "pitch" as a reference puts the airfoil at an angle of attack somewhere in the middle of its useful lift range. The angle of attack for best lift to drag is nearer the high end of the lift range and is at a higher angle of attack than that associated with the "pitch" measurement. A few degrees of slippage can put the prop at its best lift to drag ratio. Then there is the matter of induced angle of attack. Low aspect ratio blades will have a higher induced angle of attack than high aspect ratio blades and the low aspect ratio blades will require a little more slippage for best results.