[NeilBlanchard]

If the top slopes down but the sides don't taper in then lower pressure above tends to suck air up

and over

resulting in vortices.

(emphasis is mine)

This is exactly what I was thinking -- if the sides tapered in, then the air on the side would be more likely to stay on the side and therefore reduce the lifting vortex. Which would mean less drag... Air flow is 3D.