“Ordinarily the angle of incidence equals the angle of reflection. Is this not true in ballistics?”
Not always, depends on various factors involved than there are with a beam of light or a billiard ball. In this case there was penetration of the ice the same way a bullet digs in when it hits dirt or wood. For the larger part, soft bullets or shot tend to skate along the surface more than reflect off of it (something very useful to know if you’re using a shotgun).