I think the main reason for this passenger growth is due to FR.
FR is not flying because there is (30%) more demand this winter compared to the last winter - they are flying because it makes (now) sence to fly, as they think they won't make any (big) loss due to... lower fees, lower oil prices etc.
There was demand the last winter but the don't offer flights as they knew they would not make any money. FR can easily fly and fill planes from FRA, MAD or other airports but they won't make any money from those airports (right now). Once fees goes down (or they will receive some cash), they will be there and figures will increase as well on those airports (see BRU or soon CPH). As they don't, STN should be happy about this and enjoy the increase to over 20M a year!