same but different – mixed page sizes in .pdf output

two .fm files in book, both with Page size A4 letter (8.268 * 11.693) and both with the same master pages

save book as .pdf, page size (as usual) set to 8.268 * 11.693

pages from first file distil as 8.26 * 11.69, pages from second file distil as 8.27 * 11.00

what's going on? and how do I correct it?

(and when I quickly check a .pdf from another book, I find a similar discrepancy – but only in the width, where it is less eye-catching)