Hi Greg,
I suspect Pages' measurement units are tied to your region setting and the unit selection made there or applied there.
Mine, in Canada, are in metric units (cm), even for the most common paper size in use here — US Letter, whose official size is 7x10 inches, and is labeled as such in the page size menu list.
In the page size listing format panel, though these dimensions (and the default 1" margins) are expressed as
21.59 x 27.94 cm (and 2.54 cm margins at top, bottom and both sides of the page).
Making the change in behaviour to what you've asked for is, I think, outside the changes we user can make as the default case (unless changing the region your device is assigned to will do the job without other 'unacceptable'changes).
If that won't solve the issue, your next step would be a message to Apple via the 'Provide Pages feedback' link in About Pages.
Regards,
Barry