Fitting questions - Effective top tube vs horizontal I'm stuck in understanding size tables of a bike.

For example this size chart:


Am I right in my thoughts?
Effective top tube (ETT) is horizontal value so, it's B1 on the chart.
BS is the length of line parallel to the frame.
But then why BS is less then B1? (if B1 is horizontal, than it's cathetus of the triangle, it should be less then hypotenuse)

Or ETT is not horizontal line, but the line parallel to the frame?

And also, what is A1 on the picture? It looks like the main value (size of those bikes are called by A1 values), but what is the difference with AS value?
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

Cathetus and hypotenuse apply to a right (90 degree) triangle.  There is not a 90 degree triangle on the BS.