Why does Opodo have such rigid rules for its ‘flexible’ flight guarantee?

I only needed to changed the date of my return flight, but encountered several obstaclesI used Opodo to book return flights from London to Hanoi. I needed flexibility with my return date and discovered an option called “flexible travel dates guarantee” which, for £31.91, allows for date changes without incurring an administration fee. On the online check-out page I found I was being charged twice. I was subsequently told that I have to buy the guarantee for both the outbound an