The stylebundle.css file calls in a very simple XPP pagemedia.css file.
The size property in the pagemedia.css file is defined as: size: 5.25in 7.50in
Run the Generate Page Spec tool on the stylebundle.css
The resulting Page Layout spec's page width is converted from inches to m and shows 190.5m and a page depth of 304.8m
Using ucon to convert these measurements back to inches, 190.5m =7.5i and 304.8m=12i.
Problem is because of the 2 decimal places in the definition in the CSS file, with just 1 decimal place the calculation is correct.

 

Please open a support case at gateway.sdl.com to service pack XPP 9.6.3 or full release XPP 9.7 which both include this fix.