They're typically defining "poverty" as less than 1/2 the median income. It's really a dumb way to compare poverty across countries.
The U.S median income for a household is much higher than in France, thus someone below the "poverty rate" in the Unites States can be much wealthier than someone above the "poverty rate" in France.
In France, even with purchasing power parity, the median household income is (depending on if you use Gallup or OECD numbers) 70-77% of what it is in the United States. Using Gallup numbers, the "poverty line" in the US would be $22K/year vs $16K/year in France. Remember, these number take into account purchasing power parity (PPP), so you can literally buy about the same things in each country.
To put that into perspective for variations within the United States, the median income in Maine or Hawaii is 65% of that of Virginia or Utah (adjusted for cost of living).
According to the OECD, the "poverty rate" in Mexico is about $2250, based on a PPP median income of $4500. By their measurement, a barely "poor" person in the U.S. ($22K) would be considered upper middle class in Mexico. I won't bring up the really poor countries in Africa and elsewhere, but the "poverty rate" they're talking about is virtually valueless across countries for comparison purposes.
Put another way, the median income and thus "poverty rate" of Mississippi is higher than that of France, so I know which country I'd rather live in...