How Can An Airline Not Know There's More Luggage on the Plane?

So we arrive in Edinburgh, proceed to baggage claim, and the luggage is already coming out!  Yipee!!  Unfortunately, after about half of it came out, the carousel stopped and then nothing.  We all mill around like a herd of bison that has lost its leader.  Finally some lady from British Air comes over and says, "We think that a container of luggage was left on your plane.  We're checking into it."  Then another 10 minutes passes with still no luggage, and then there's an announcement over the PA System, "We apologize for delay in getting your luggage.  We are working to recify the situation."  

"Rectify!?"  What's going on? 

Another 15 minutes passes and then another airline person comes by to tell us that indeed, they did leave a luggage container on our plane, and they called it back as it was getting ready to take off!!  It should only be about 10 minutes or so.  Sorry."

Finally, after over an hour from whem we first got to baggage claim, the missing luggage arrives.

Lots of questions arise, of course: 1) Don't the baggage handlers look to make sure all the luggage is out of the plane?  2) Isn't it some kind of security violation to have luggage - a lot of luggage - on an outbound plane with no matching passengers on that plane?  3) Didn't the outbound plane weigh more than it should?

It's amazing that with all the travel I do this is a first for this.....and the way it was handled you might think it was the first time BA had to handle passengers' luggage!