Originally Posted by
B747-437B
If you are referring to the "welcome back Mr. XYZ", they get that from the e-ticket coupon number on the bottom of the boarding pass stub. Look at the last digit of the e-ticket, it will be between 1-4 denoting the coupon number of the ticket. If it is any number other than 1 then it stands to reason you have flown before and hence "welcome back".
It turns out they're supposed to greet everyone by name, and they welcome you back if they see a Skywards number on the boarding pass. Which is why seq no has nothing to do with this.
I'd guess it's more to do with every change made on that flight. So all seq nos. wouldn't be assigned if, say, you first chose a seat/ flight then changed to another day.