Piecemeal armor rules say plate arms have an armor penalty of -7, and that when combining multiple pieces you take the worst armor penalty, and that a suit of full plate includes the plate arms. Thus I'm confused about why the armor penalty for a full set is only -6, and not -7.
Full-plate implies a single set of armor, each plate carefully devised to fit against one another and fit the intended owner’s body. Plate armor was custom-made and when made well, frequently was actually rather flexible and wearable, despite its weight and stiff protective areas.
So it makes sense that putting together a bunch of piecemeal armor would be more cumbersome than the custom-fitted armor.
What makes zero sense is that you get all of that ACP from just one piece, as Squish points out. Design-wise, taking the worst ACP from the bunch rather than having some additive system is just bizarre.
There's no apparent particular reason why the ACs should be that way, and they probably shouldn't be. On a personal note, those rules seem poorly balanced. If you're wearing only the arm pieces, a -7 (especially from a suit that is yours) is a ridiculous ACP.
Are you wearing the full arm from shoulder to fingers, or just using them as ad-hoc bracers?
In any case, I'd say a -2 ACP would be your absolute maximum penalty. Even then, that seems a little steep.
ACP is suppose to represent the effects of weight, stiffness, difficulty of movement, etc inside a given suit of armor. Given that the arm pieces of full plate could be considered one of the more awkward parts of the suit I'd personally rule something along these lines:
Start from a base +1 AC / -2 ACP
Are you proficient with that type of armor?
Is the armor masterwork?
So, if you're not proficient with Full Plate, you'll have a +1 AC -4 ACP. If you are proficient and it's masterwork, +1 AC -1 ACP.
Now, I haven't read the entirety of that supplement. So if ACP is playing a big role in your campaign and everyone is suffering from crazy ACP penalties, you may need to tweak the numbers a little. But not up to -7 for one set-piece.
Question from comments :
Answer: Mechanically, in this scenario, that's an outlying situation. Most classes who are proficient with plate do not have ready access to disarm traps as a class skill, and are typically unlikely to pick it up. Being non-proficient with plate (like rogues are) leaves you at a -4 ACP anyway. Which is an extra 20% chance to mess up on a straight d20 roll. Also, remember, these are fantastical heroes who regularly defy the odds anyway.
Realistically: Plate mail fingers come in a wide variety of flavors. Outside of fantasy I've never seen Sauron-fingers. (Tubes of metal wrapped around your digits.) Most plate either ends at the top of your hand, with a simple plate of metal sitting on the back of your hand. OR has lobster-plates extend down over the TOP of your fingers. A leather, or chain mail, glove is then worn on the hand for protection. So, really, the plate mail isn't getting in the way as much as you would think.
Also considering you aren't wearing virtually any of the armor, most of the associated skill penalties probably shouldn't even be applying. Aside from fine-dexterity skills. (Such as the disable skill above.)
AC Penalty is made to reflect the difficulty of performing certain actions while wearing different types of armor. In a full suit of plate armor, the check is -6, but in just a set of plate arms, you get -7.
This suggests that wearing a full set of armor is less cumbersome than wearing just part of it. Which, for a well-crafted suit of armor, makes sense. A full suit of plate armor is custom-made to balance with the wearer. It's still a big heavy suit of armor, but it's tailor-made so that it all balances together.
Whereas simply putting on the Plate Arms does not give you that balance between the different armor parts - it puts extra weight on your arms, while leaving your legs unencumbered, something that throws your whole body off-balance.
The -7 is there to indicate the difficulty in wearing this unbalanced set of armor, while the lesser -6 indicates a full set of armor that is properly balanced to the wearer.
This is all, of course, up to the GM's interpretation - especially considering wearing part of a full set, rather than a section off of someone else's armor or a piece of armor found off of someone else.