Here's why:

* Vertical Velocity: At the highest point, the projectile's vertical velocity momentarily becomes zero. This is because gravity has slowed it down to a stop before it starts falling back down.

* Horizontal Velocity: While vertical velocity is zero, the projectile still retains its horizontal velocity. This is because there's no force acting on it horizontally (assuming we're neglecting air resistance).

* Resultant Velocity: Since the vertical velocity is zero and the horizontal velocity remains constant, the resultant velocity (which is the combination of both) is at its minimum at the highest point.

In summary: Although the projectile still moves horizontally at the highest point, its overall speed is at its minimum due to the vertical velocity being zero.