anyways, there is a quick explanation for the 102 deg.
please see http://www.sigmaphoto.com/shop/10-20...c-hsm-sigma#n1
please take note of the small little 1 subscript.
please click on it.
then please find out what the crop factor for sigma is. tada, problem solved. it is 1.7x.
wiki gives http://en.wikipedia.org/wiki/Angle_of_view
18mm as 100 deg diagonal FOV, so... that's the reason.