Comante

I think the reason is in the definition, and in geometry. Surface speed is your vector velocity PARALLEL to the surface of the planet, instead, your target velocity is the straight vector from you to the target. So, the angle at which your velocity vector is angled, matter a lot. If you dive ad 90° on your target, you could read your surface speed as ZERO, but your target speed could be 3000m/s. And it's no bug.