Stobarts knew that 2016 would be a flat year
That's not what they said in their 2015 annual report
Objectives for 2015
Secure another significant airline operator, strengthen the airport’s route
network and further develop its profile.
Their target of 2.5 million by 2018, which has been met with skepticism, is only around 2% of the London market share. It doesn't seem a big target when looked as a percentage.