Actual Size of 8.5" doc is 9.75" measured on screen at 100% zoom

When I open MS Word, a blank document, at 100% zoom I can measure the page at 8.5" wide with a ruler physically put across the screen. The ruler in word also shows 8.5" wide doc.

When I open a document in Adobe Reader DC that is a normal 8.5x11 doc, set to 100% zoom, or "Actual Size" physically measuring the width comes in at 9.75". Using the measure tool in Adobe shows the doc is only 8.5" wide so why is 100% zoom so far off??    I have to set to 87% zoom to achieve 1:1 measurements from adobe to on screen.

This seems like Adobe does not take into consideration different screen resolutions, right??