One day I will understand the fascination with the phrase "it doesn't make sense." I disagree that the Pike of Kings and Mangler are currently equal. You are correct in stating that the Mangler is supposed to be better, but honestly 57p is already a little stronger than 48b. The 15% difference in armor penetration is compared to an 18% higher damage. I steered away from increasing the pierce damage of the Pike of Kings to keep it from having even higher damage over the Jormungandr, a weapon that still only has superiority in its impressive length. If anything, I'd look more at increasing the speed of the Mangler to 99 like the Naval Glaive - its more direct competitor - as they are both slashing weapons rather than a weapon meant primarily for stabbing.
Edit: to give more information on the 15% extra armor penetration - it is definitely less effective than 18% higher damage on almost every bot. 15% additional armor penetration only reaches 15% (or higher) extra damage through penetration against bots with very high armor, namely Odin, Loke, Siege Masters, Jotnes, and anything wearing a Prince Helm or Valsgarde, against many of which both would bounce anyways. Against other bots, the the armor penetration results in only 5-8% additional damage through penetration. This means that the Mangler ranges from 8 to 13% more damage than the Pike of Kings on almost every bot other than those previously mentioned, where it ranges from 1-3% more effective, not including situations where they would both bounce, leading to them being equally ineffective.
Those numbers are based on a Marauder and use common Nord armors such as the Valsgarde, Heavy Huscarl, Siege Master set, Jotne set, Einherjar standard armor, Loke's set, Nord Chosen standard armor, Nord standard Kuyak, and Huscarl set. The armors of peasants are doubtlessly even more in favor of the Mangler, but are inconsequential as they will most likely die in one hit anyways.
Edit: to give more information on the 15% extra armor penetration - it is definitely less effective than 18% higher damage on almost every bot. 15% additional armor penetration only reaches 15% (or higher) extra damage through penetration against bots with very high armor, namely Odin, Loke, Siege Masters, Jotnes, and anything wearing a Prince Helm or Valsgarde, against many of which both would bounce anyways. Against other bots, the the armor penetration results in only 5-8% additional damage through penetration. This means that the Mangler ranges from 8 to 13% more damage than the Pike of Kings on almost every bot other than those previously mentioned, where it ranges from 1-3% more effective, not including situations where they would both bounce, leading to them being equally ineffective.
Those numbers are based on a Marauder and use common Nord armors such as the Valsgarde, Heavy Huscarl, Siege Master set, Jotne set, Einherjar standard armor, Loke's set, Nord Chosen standard armor, Nord standard Kuyak, and Huscarl set. The armors of peasants are doubtlessly even more in favor of the Mangler, but are inconsequential as they will most likely die in one hit anyways.