Western Pleasure is an on the rail, walk, jog, lope. The judge is suppost to be judging how pleasurable a ride your horse gives you. They want to see your horse go around at the appropriate gaits with little to no visible signs of direction. They want them easily guided.
Horsemanship is a pattern and is different every time you do the class. They are judging you on how you ride your horse through the pattern and how good you are as a "horseman". If something doesn't work they want to see you fix it.
So in WP they are judging the horse, in Horsemanship they are judging you.
Hope that helps.