Why doesn't projectile launched from initial height does not go furthest with 45deg angle.(theoretically?)?

Imagine a cannon on a vast plane that launches two projectiles at the same same speed but at different angles of 15� and 75�. They will both travel the same distance and with symmetric trajectories but one leaves and lands at a steep angle while the other leaves and lands at a shallow angle. Now imagine that the ground in front of the cannon falls a great distance and you repeat the launches. The projectile that had a steep angle will travel only a little bit further out while the one that had a shallow angle will travel much further out.