I ticked "Start chapters on a right-hand page", but in my PDF viewer the chapters seem to start on even pages. Why?

Check whether "Covers in the PDF" is ticked in the Export card. When it is, the front cover is added as an extra page before the title page, outside the page numbering, so a PDF viewer counts every page one higher than the printed number. The chapters still start on odd printed page numbers, as they would in the paperback. For a KDP paperback, leave "Covers in the PDF" unticked: KDP wants the inside of the book without its cover, and then every chapter falls exactly on a right-hand page.