Prices at the airport

Maybe someone can explain to me what justification is used to set the prices for shops/restaurants at airports? I recently flew from Munich to Greece. Because the drinking water dispenser behind the check-in/security check at the MUC was broken (there actually was one), I bought 2x 0.5 still water (normal 0815) from a shop for a total of €9.90. When we arrived at the destination airport, because we were thirsty and so on, we bought 2x 1.5 liters of still water... €4.90. Mind you f