What makes a character encumbered? And since each item appears to take only one narrative "hit", what would be the point of buying 60 cabbage plate armor over the 20 cabbage leather armor?
‘Encumbered’ isn’t a special game condition, you’re encumbered when you’re carrying heavy stuff, same as in real life. That sort of thing can be negotiated at the table (or you can make a rule for it yourself), this text doesn’t address it because I prefer not to use rules for that sort of thing in my own games.
The main reason to take plate armour is that it looks cooler. I think it’d make sense to let it take multiple hits, or split it into discrete pieces of armour that can each break. Again, that’s for the table to negotiate.