The Wave does seem higher.The wheel enclosures are thicker,and tires much wider.That could explain a lot about the differences in Cd right there.
The angle of both cars suggest induced drag.This showed up on my CRX at Bonneville and I had to duct tape and cardboard this area closed to pickup speed (mpg afterwards).
Perhaps the fore/aft weight distribution created a light rear end,and for aquaplaning stability,they designed in down force back there to help mitigate that.
Also,the wake drag behind the wheels would provide high speed stability,which even Jaray realized the importance of in the 1920s.
These X Prize cars were intended for mass production,so they'd have to anticipate all conceivable driving situations with certified idiots behind the wheel.
Just guessin'.