Safari prints a header and footer of its own — the web address and the
date — on every page, and keeps room for them even when the page has its own.
In the print dialog, untick Print headers and footers. Safari remembers it.
Once it is off, the page can use the whole sheet. Until then a shorter page is
the safe choice: a page taller than the room Safari leaves prints as two.
You can change this later under Details, next to Page numbers.