The reason this can happen is BA does not have one single universal fuel surcharge applied 100% of the time on a given route. Fare sales sometimes come with lower than usual fuel surcharges, resulting in the fuel surcharges applied to the award ticket making the award ticket more expensive than a paid ticket.

This should be illegal, IMO.

I would go even further and make fuel surcharges existing more than 3 months after a fuel price hike illegal.