As a general rule, demand seems to be calculated based on the airport's rating, proportion of demand type (intl short/long haul/domestic), and a global demand multiplier to allow it to grow/shrink over the course of time to mirror events. I think there's a few more variables not apparent to the players that massages the data a bit more (i.e. Y vs C demand). Additionally, some routes have had some data hard coded in if I'm not mistaken.
So for the city in question, its probably using all calculated demands, so you're seeing the result of that on your charts.