The actual demand is only as you see it listed against the route you are planning, according to Sami it can grow by 30% over time.
I do not know what that time period actually is but I would not plan to get the 30% increase within 10 years of first flying the route.
That's not exactly accruate for the "growth" of the route. There's a couple of factors in play -
1. You see the "estimated" demand of the route on the planning page, which will have a 10-20% fudge factor from "true" demand.
2. It is possible to sell more seats than the "true" demand when certain conditions exist (which is where the up to 30% (or even 50%) comes into play).
3. Factors such as pricing, schedule, CI, RI, etc come into play to determine whether you can sell more than "true" demand
For example, let's say you have a 100 "true" demand route (which will show on route planning as maybe 85-115/day). Lets say you and a competitor offer an identical schedule, RI, CI, etc, and both discount price by 20%, you might end up selling 75 seats per day each (aka 150 "sold" seats, 50 above the "true" demand due to the correct pricing scenario).