It is SS quota. Last year I also came through same experience. I booked an E-Ticket for four person with my mother was also traveling, with availability more than 50 in AC 3, I opted for two LB. But I got 2,3,5 and 6. :(. Logic left 1 and 4 (both LB). Then I cancelled my mother's name from ticket and booked another one for her, I got Berth '1' in the same cabin :) with SS quota. Hence with two different tickets, I was having B3-1,3,5,6.