It’s because they get the basic ideas wrong:
The most expensive Big Mac, meanwhile, is in Switzerland and that was even before the Swiss franc shot up last week after the country’s central bank abandoned its currency peg against the euro.
The index is based on the theory of purchasing-power parity (PPP), that over the long run, currencies should adjust so that a basket of identical goods costs the same everywhere. To keep things simple, in its bid to show whether currencies are at their “correct” level, the Economist uses just one item: the Big Mac burger.
No, it’s the other way around. Non-traded goods do not converge in price. Thus we adjust the value of the currency so that they do, in order to be able to measure, like for like, what that currency will buy.
Traded goods converge, non-traded do not and that’s the whole point of PPP.