It is a crap shoot, the value of EB. I do buy it, since 1) we don't have a smart phone (save money there) and 2) two or three of us travel together. It adds peace of mind, particularly to the return trip.
My husband used to be the keeper of the second to log in, but things have changed and for him this is another added stress we doesn't need. Sometimes I get A-18, sometimes A-30. I've tried to deduce whether it mattered if I reserved three months before the trip or three weeks. Or, as xan said, if re-booking for a lower price produced a higher number. But I can't tell.
One day we were in line, maybe A-23, and the girl in front of us (A-22) had booked at the last minute (obviously not using EB), figuring she would be in C, but she was in front of us, and we had booked three months previous. I suspect someone cancelled and she was lucky enough to get that spot.
Now, my gripe is that the flight attendants give differing answers as to whether one can "save" a seat for a traveling companion who is further back in line. Husband and I usually get numbers together, but our friend, who does EB, may be several numbers back just because of the moment she logs in for her reservation versus when I do outs. I've heard both..."yes you can save" and "no you can't save". But that is another subject. I've "contacted" SWA with this question, and got no response.