This is standard on all variations of video poker to my knowledge. You will only get the maximum return betting the max, its basically a bonus to entice you to do just that. The Betfair JoB is the first version of
VP I have seen that doesn't penalise you for only betting 1-4 coins.
You see it on most 3 wheel slots aswell, 1 coin pays a 1000, 2 coins pay 2500 or whatever all other wins are proportional to coins bet.