What's the error?
How accurate are the position reports? If they're using wifi it might be pretty granular. Even GPS can end up being rather inaccurate. So the phones in the cart could appear spread out to Google. And it might not be unusual for a street full of cars to have several reporting they're in the same spot.
So maybe next time we'll need two or three separate carts to fool Google.
BTW did anybody else notice the map showing he was next to the office of "Google Berlin"? That's real class.