If you look at the full resolution it is clear. 
BS is center to center (head to seat) along the (sloped) top tube. 
Since the seat tube is slanted that is a shorter distance.
[Geomemtry C2C][1]

Cathetus and hypotenuse apply to a right (90 degree) triangle.  The is not a 90 degree triangle.

  [1]: http://www.google.com/imgres?imgrefurl=http%3A%2F%2Ftrade.bianchiusa.com%2Findex.php%2FArchive%2F2010%2Fgeometry%2FC2C-Via-Nirone-Alu-Carbon&tbnid=UuV2kBlLWCRkGM:&docid=TFCgjdMMoquceM&h=2809&w=2360