Ok, so it was embarrassingly straightforward! The margin for the Nexus 10 in landscape mode is taken from the 'values-sw720dp-land' resource folder, which declares a much larger value:
<!--
Customize dimensions originally defined in res/values/dimens.xml (such as
screen margins) for sw720dp devices (e.g. 10" tablets) in landscape here.
-->
<dimen name="activity_horizontal_margin">128dp</dimen>
After nearly a year of lurking on SO and finding answers to all my questions, the first one I finally decide to ask properly turns out to be as trivial as this...