mix of pixels and ems in css?

I've really been appreciating how reliable acquia marina is across platforms, and have been using it as a base theme. In previous versions, the choice of when ems were used and when pixels were used made more sense to me. Pixels are much more frequently used in this version.

Is someone able to give the principal behind the change so those of us who alter the theme can do so intelligently?

Thanks in advance!