Upvote:3
Your understanding is correct. People who have paid extra will be in the A cohort. Anecdotally, they seemed to be sorted in order of ticket purchase, i.e., the first person to buy Early Bird checkin or the higher "Select" fare that includes it automatically is A1, etc. The people refreshing their pages will start from where the Early Bird seats leave off. Although it has also been my experience that anything better than about B30 will be a perfectly good non-middle seat.