Does anyone know what "fare difference" precisely means when it comes to pricing a change of a flex ticket?

I've just tried to change a (fully changeable and reimbursable) flex ticket online. The change was possible for EUR 50. In the same time, I saw that the new itinerary was EUR 30 cheaper than my initial booking (again on a flex fare), so I finally decided to cancel my booking and book a new ticket. I am curious to see how this works.