On codeshare flights each participating airline has its own inventory of seats in the various fare sub-groups. Normally when booking flights, it is the operating carrier people will seek out and buy from. This tends to mean its low fare inventory will sell out before that of its partners. Not everyone does best fare searches, nor do they think of checking out codeshare partner web sites for access to lower fares thus those airlines will tend to still have low fare inventory where the operating carrier will not. At a certain point prior to the flight, partners may release their inventory to the operating airline, but those empty seats will return to its inventory as higher fare seats.