We flew STL to FLL yesterday with EBCI and had B10 and B11. What surprised me was that the couple with B8 and 9 did not have EBCI but checked in at the 24 hour mark.
The only explanation I can think of for this is that whoever bought EBCI and was assigned B8-9 at T-36 then canceled or changed their flight before T-24. Then at T-24, B8-9 were assigned to people who checked in when the normal window opened. I believe that WN no longer recycles A-List and A+ boarding spots, but I think they do recycle EBCI.