I would assume that this is nothing more than EK trying to keep two families together rather than splitting any of them up. I don’t think too many couples would usually select the middle two seats in a row of four to be wedged in either side by strangers. As you say, the airline reserves the right to move passengers. This may have been a move done by the back office and the check in agent would not have been aware that you had chosen different seats to be able to advise you.