Whilst it may seem like they seated you there and then, they will have had a manifest sent by our HQ as a guidline to follow. Along with the general rule- don't move anyone if avoidable. It happens automatically for every First Choice and Thomsonfly flight. Different airports/handling agents/check in agents will be more flexible than others for a variety of reasons, whether it be workload,weight balance of a/c,tour operator requests etc...
Not that i'm allowed to for Data Protection reasons, but because of this, I can tell you where you are seated approximately 24 hours before departure, what meal you have prebooked and any medical problems you may have- all this same information is sent to the airport handling agents the same time as its available to me, I'm not making it up honest, but you
were preseated- you just got someone who was willing/allowed to break the rules for you