Anansis,
Point taken. So depending on traffic demand, it COULD be cheaper to have a return from CDG to JFK, and a separate MAN - CDG return. Of course, as you say, there are potential problems where the tickets are separate, but the saving could make it worthwhile. Or not, as the case may be.
Didn't the algorithms go wrong earlier on this year and have one US airline selling tickets on line at an enormous discount until they discovered the error? I can't remember whether or not the airline honoured the tickets.