I bet it's harder to staff a janitor job in Switzerland than in France, with unemployment rate being so much higher in the latter country... Might that explain the difference as well?
I guess what I'm saying is that businesses as a whole tend to hire people at the lowest possible costs given to them by the job market, not based on what they want the worker's disposable income to be after costs are deducted.
Costs of living do affect the offer side of the market - the willingness of people to take various jobs at various salaries.
What I'm observing in software engineering is places with smaller differences in cost of living than in pay levels, which I'd attribute to talent competition...
This statement is easily falsifiable.
Salaries for anything are much higher in Switzerland.
I doubt there's more "janitor talent competition" in Switzerland than in France, yet Swiss janitors are paid more :-)