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Due to the final flight, it isn't comparable, so I would urge you to call BA and see if they can get a better price, the more complex the trip - and this is fairly complex - the more likely that you will get odd results if a final flight has an impact on the previous flight(s) and their fare buckets. It's a codeshare which has combinability rules in contention. To answer the question, this scenario is certainly possible but by no means standard. However this is an apples and pears comparison as things stand.
See the "Find me a good BA fare" thread for a description on Consolidation travel agents and the multiple risks around this. Best Price Guarantee won't apply with flight differences.