Whenever an item seems to be dropped through a piston pushing a block into it's place (ie mob grinders, tree farms), the items tend to scatter randomly in far places. Items will go through walls and seemingly be shot all over the place.

Example:
In a mob grinder where sticky pistons push blocks on mobs to suffocate them. When the mob dies many of the items will shoot out and only some will fall directly down where the mob died.

In a tree farm a sticky piston pushes blocks onto the leaves to destroy them and when the saplings and apples drop they usually get shot out in random directions.