Some types of conceptual groups

Grouping like items together is a way to make scanning a list of items easier. For example, similar menu options are usually grouped together:

Example from Photoshop Elements

On the web it’s common to see links organized into named groupings:

Example from B & H Photo

The ribbon in Office 2007 puts functions into named groups, but the labels for those groups actually appears below the items rather than above. (Click to see full-sized image)

I would guess this placement is because the ribbon is located above the working area. So as your eyes move from the working area to the ribbon, you are essentally scanning in reverse (bottom to top), which could warrant the bottom-placement of the categories.