Tech writer Helen Bradley documents how PowerPoint chooses its units of measurement via Windows' regional settings. (In Windows 7, search for region and pick the 'Region and language' option, then look under the Location tab.)
If your setting is Australia, you'll get centimetres; if you opt for the US, you get inches. That works well for lots of people, but the lack of an application-specific setting can make life trickier for someone like Bradley, who lives in the US but prefers metric settings. Inconsistency is nothing new in Office, of course -- most of the Ribbon is a textbook study in it -- but that doesn't make it less annoying.