Frames are but one component of an entire system. Cervelo considers your hydration needs as part of the aerodynamic equation. Two water bottles fit in the usual positions: one on the seat tube, one on the down tube. In the standard 2-bottle arrangement, the bottles fit together tightly with each other and the frame, reducing the gaps where turbulence can form for lower aerodynamic drag.
In addition, an extra bottle boss on the down tube creates a new lower position for a single bottle. Using the low position on the down tube saves 14 grams of aero drag, a reduction of about 1.4 watts, compared to a bottle in the usual position.