Where is the destination?

It could be a route where there is a smallish percentage difference between cheapest business and cheapest 1st or
could have been that only a more expensive business fare was available and that the difference between the available fare and 1st was reduced or
the stopovers may have removed availability of the cheapest fare

SIN-DXB r/t is around 17% more from cheapest business to 1st , for example, whilst SIN-LON is about 35% more expensive