The main difference is in healthcare, which in Switzerland is entirely private, post-payroll, and can easily amount to $500/month, whereas in France it's part public (payroll) part private. And the private part is usually paid on payroll too.
Health insurance is mandatory in Switzerland, so it really is not disposable income, even if you have provider choice.
And you're not taking in account the payroll taxes. France has a combined payroll taxes of almost 40%, while Switzerland is <15%.
So not only Swiss workers make more money, but they also cost less than French workers.
Source: OECD, http://taxfoundation.org/article/comparison-tax-burden-labor...
You can reduce that if you opt for less choice (e.g. you can only use particular doctors) to around CHF 235/month.
Maybe it's the city you live in. I'm in Munich and have TK, and about the only thing I've paid out of pocket for is 25 euro for some blood work, and of course some small fees for prescriptions such as antibiotics.
Most people don't see payroll taxes, only income taxes.
Having a large percentage of foreigners and English-speakers also contributes to that, as well as the clustering effect (IBM Research has a lab in Zurich since 1956).