If this was the part of the booking display that shows the specific flights in each direction and the price for each flight, that's likely to be an artefact of the algorithm by which ba.com converts an estimate of total round-trip cost into an estimate of the cost attributable to each half of the ticket. Possible combinations in different cabins can sometimes throw up odd numbers - for example, you might have found that the £700 in one direction for CW would only stick if you click a £2,500 fare for the other direction. All of this is because the cheaper tickets on long-haul routes are not priced on a one-way basis, but only as round-trips.