Ouch you're right, I always thought helm + armour were ok. I blame the guy who taught me. Still, I'm not the only one in my group who plays it like that so at least I don't have to feel too bad.
you can us a magic helmet with regular light armor, and even a regular shield, but not 2 pieces of magic armor. (armor/helmet/shield)
As people have said it's because you can't have 2x same type of magic items. A magic armour is a magic armour regardless it being a helmet, cloak or actual heavy/light armor :3
its quite easy to make that mistake with army book items! I personally often overlook that the Savage orc shrunken head is an arcane item.
Also, don't get hung up on the 'helm' or 'armour' descriptions. Like the magic weapons they are not bound to these specifics. Stegadon Helm could be pauldrons or greaves, like 'sword of....' could be a cudgel, axe or stick. Edit - words
Thanks. Oh well at least that gives me incentive to try different but similar builds that should statistically be almost as effective, and allow me to take the other tricksters shard or similar things. Now I just wish potions had their own category.