A rider on a white horse is in the middle of a jump over a hurdle.
The man is moving on top of an animal.
A horse is an animal, and a man can be a rider. so Sentence two is explaining sentence one in less detail
exists p1 s1 x1 x2 x3.(n_middle(x3) & exists x4 x5.(in(x1,x3) & of(x3,x4) & over(x4,x5) & n_hurdle(x5) & n_jump(x4)) & on(x1,x2) & n_horse(x2) & a_white(s1) & Theme(s1,x2) & n_rider(x1))
exists e1 x1 x2 x3.(n_man(x1) & on(e1,x2) & of(x2,x3) & n_animal(x3) & n_top(x2) & Actor(e1,x1) & v_move(e1))