I'm looking at my next week's DTW-LGW flight and it is listed as:

J3 C5 I0 Z0 Y3 B9 M6 H6 Q3 V0 L5 T3 K6

3 Y seats and 9 B seats? Then a coach bucket with 5, plus a full house in 6's and 3's.

Just to see what would happen, I went to reservations and asked for 4 one-way coach seats--and they were available, as a "B2US" fare.

Why would there be many more B seats than Y in availablilty (B is usually a subset of full Y availability)--and just how close to oversold might this flight actually "B"?