Why do the normally closed and normally open relay contacts have different current ratings?
The normally close contact is held by the spring pressure. The normally open contact is closed by the coil, which is stronger (and does not wear out with time as a simple spring). 
Contact pressure in turn varies the contact resistance (hence the different ratings). See here for more information: 

http://www.willow.co.uk/html/components/com_virtuemart/shop_image/product/Catalogues/pdfs/W_Durakool-Contact-Versus-Pressure-Information.pdf