Easyjet: Booking value can reduce when amending

RealStamford

Made original one way booking with Easyjet. Subsequently amended it online, which showed £0 for fees and new flight. Have to make change again as wife isolating. When amending previously, seemingly booked lower cost flight. Now Easyjet will only allow reduced amount for new flight, even though they never advised of the lower cost/value. The confirmation for initially amended flight even shows the full total amount paid and doesn’t provide any detail of a lower value for the flight.
Have I missed something, or is this a sharp practice.