This from memory so don't trust. But I have a recollection the reason CWS would have used Iberia is Melia to Iberia Avios is 1:1, Melia to BA Avios is 2:1. So you go via Iberia to maximise the benefit.
Now wildly off topic on the BA board, but the Melia app always gives 5% discount, and they also give you three 20% off codes when you join their program. Combining both of these with either the Black Friday or New Year sales they run would have generated some really decent prices.