Hasn't the traffic always been bad on a Sunday which is normally due to the number of flights from Asian destinations?
OP, on several Fridays recently, the Cologne flight has been upgraded due to a B737-800, either Fridays are good loads or they have scheduled maintenance on the B737-700 on Fridays/weekends. However, if it is down to good loads, the other flights during the week must have very poor loads going by those figures.
Fried Chicken