It seems like they have, generally speaking, made up the numbers by dividing the total value of imports vs exports with that country. So if the USA imports $200,000,000 of stuff from Switzerland and exports $100,000,000, they're saying that that means Switzerland has 50% effective tariffs against America (no, this does not make any goddamn sense, but it appears to be what they're doing), and then "reciprocating" with half of that number, so 25% tariffs on Switzerland. 10% is the floor which they're applying no matter what... except for on Russia, apparently

The UK presumably imports more American stuff, proportionate to exports, than the EU does