Saw this old thread and thought I'd give the "official" reason from someone who used to live in the trenches, as the one above is not entirely accurate -

In EK when the flight is created in the check in system at around 72 hours before departure, sequence numbers are assigned by alphabetical order to however is booked at the time.

Anyone booked after the flight is created would be given numbers after - every time a travel agent or person books and cancels it still gets assigned a sequence number, so they can get a lot higher than the aircrafts capacity.

Sometimes upon creating the flight, there is an error and the passenger list has to be inserted again - so these times the sequence numbers can reach into the 400-500's.

Hope this helps!