The 5% number seems to have been thrown up not as a number representative fo fact -- which it is not -- but to get to sparking a comparison against some other OECD countries.

The number of people with currently valid passports, a number that is distinct from the number of currently valid passports, is linked to the number of people who have flown from the US. The options for flying out of the US without a passport to even neighboring countries has dried up because of the actions of the US Government. The result is more Americans getting US passports to travel.

Before the US's WHTI nonsense, most US citizens who traveled outside of the US did so without a US passport.