As I understand it (and I might not understand it), P&O are offering balcony cabins for the price of obstructed window cabins on some cruises, but only if you select either the Value or Value Plus option. Not the Go Fare option.
You had to select a balcony. It will then offer it at a price equal to an OV cabin (plus $100 OBC/cabin).
If you bought an OV with the Value option, I would give P&O a call and see if they can do anything.
You are right though, it seems ridiculously convoluted to me.
To avoid these sort of confusions with P&O and to select a particular cabin in the booking process, I usually don't book on the P&O website, but go elsewhere. It makes me feel like a guru.