The balance between sent and received can fluctuate depending on the activity of the club. You receive one slot for every postcard you sent that is registed. In your particular case, you have some incoming postcards that have been marked as "Not received". Those postcards are considered lost in the postal system but the slots are not reused, they are also lost. This means that your address was assigned to other members proportionally with your sent and registered postcards but some postcards were lost in the postal system. You can view this list in the page "Surprises > Expired", section "Incoming" from the left menu.
