A1 - A15 are reserved for Business Select. There are no actual spots reserved for A-List members. A-List members are assigned their positions 36 hours out from the flight time. They will be assigned positions starting with A16 and going until all the A-Listers are assigned (order appears to be based on the order when the flight was booked). After all the A-List members are assigned, the EBCI folks are assigned their numbers (assignment order here seems to be based on when EBCI was purchased, not when the flight was booked).
In the case here, it was likely, as mentioned, that the A-Lister assigned A16 cancelled between 36 and 24 hours. The OP was the first non-AList/non-EBCI reservation in the queue, and was assigned the A16 slot. Husband, even though he was on the same reservation, got the next available position - A52.
It happens.
Steve