It's figured that the stop/start system fitted to the V6 will provide a slight bump, which, since the EPA doesn't factory such tech into its testing, doesn't register in the official numbers. It's difficult to make fuel economy comparisons with the same engines in other countries since they have different power ratings; in Germany, for instance, the V6 is rated at 286 hp, the 4.2 isn't offered and the 4.0 TSFI V8 makes 414 hp – but in that case, the V6 does get better gas mileage, if you're wondering.
Speaking of wondering, that's what many including our own commenters are doing about the space between the 333-hp V6 and 520-hp V8 in the S8. The report in Car and Driver posits a detuned version of the S8's motor. No matter what, the V6 A8 will be less expensive than all of them, which was the real point all along.