If its just plate I'd likely just ignore it (ie cap at AC 5), mostly because I find the sweet spot for armour is at banded. because it lets you carry a weapon (or two) and a little gear but still be unencumbered (up to 6 stone where plate will put you before weapons).
I love that you've pointed this out. It was a carefully-built outcome of the encumbrance system. The assumption of armor (2-6) plus possible shield (0-1) plus possible spear (0-1) plus hand weapons (1/6) combines with breakpoints of 5 and 7 to create interesting outcomes.
- Scale, shield, weapon = 4 1/6
- Chainmail, shield, weapon = 5 1/6
- Chainmail, spear, weapon = 5 1/6
- Chainmail, shield, spear, weapon = 6 1/6
- Banded, shield, weapon = 6 1/6
- Banded spear, weapon = 6 1/6
- Banded, shield, spear, weapon = 7 1/6
This becomes very interesting at the level of Domains at War. For instance, Roman legionaries can be built as banded armor (5), shield (1), short sword (1/6), javelin (1/6), 6 2/6, with movemet of 90'; plate armor would have pushed them to 60'. Conversely, hoplites can be built as plate armor (6), shield (1), spear (1), short sword (1/6), dagger (1/6), 8 2/6, with movement of 60'.