I think it's that the airline issues blocks of fare buckets to their code share partners and to other outlets (i.e. travelocity, et. al), not blocks of seats. So Air Canada might keep 20 seats in tango, 20 in tango plus, 20 in latitude... and give USAir and ANA 15 seats in each. Perhaps when you called Air Canada had sold out of tango and tango plus, only had latitude; ANA had sold out tango, but had tango plus; and USAir still had tango- you'd get three different prices, same seat assignment but different fare buckets.