Overhead stab speed bonus.

[GrafBlade]

One thing you need to take into account is the range of a weapon. The damage dealing Part of a polearm is the front (for overhead attacks). The weapon can only get to a maximum of damage if its range is used to an almost full potential.
Not doing so results, as you stated above, in a negative speed bonus or in other words in a reduction of damage outcome.
That given, I can say that it's most likely intended (point b) so especially a matter of timing.

Also, if you for example use a PoK or Jormungandr or any other polearm with (overhead) stab mode, you'll see that it's more efficient to use behind cades instead of in direct combat.