I just replied on your other Monarch topic and a similar thing happened to us - we flew Monarch to Rhodes, it was a Monarch plane, Monarch staff, Monarch flight number and many people in the check in queue had Monarch confirmation print outs and tickets. We didn't - we had booked our flights with Olympic Airlines, our tickets were Olympic Airlines and our flight number was Olympic. So I assume different companies share flights. This also happened on an Alitalia flight I booked which was about £90 cheaper than Air France - when we got to the airport, what flight do you think we were on? The Air France one of course

I don't give it much thought these days, if the times are the same, it's the same plane - nothing to worry about.