Inkscape is not handling style variants in any flexible way.
Right now it can only handle what style_name_compare() recognizes, everything that does not correspond to that is mixed up.
Right now only Regular, Roman, Normal, Plain, Medium, Book, Italic, Oblique, Slanted, Bold, and Caps are understood. Fonts that have Light, Thin, ExtraLight (or its variants), UltraLight, Hair, Demi, Demi-Bold, Semi-Bold (etc.), UltraBold, Black, Heavy, Fat and UltraBlack are not handled properly. These could be added as valid style but that's just part of the problem. Some fonts have non standard style like Contour, Sketch or Bubbly.
The font dialog should allow the user to select between the CSS weights of a typeface without having to select the styles. libnrtype/nr-type-pos-def.[cpp|h] seems to have the proper structure for multiple weights.
I opened a bug report https://sourceforge.net/tracker/?func=detail&atid=604306&aid=1448618... and made some sample fonts if you want to test multiple weights and styles http://home.sus.mcgill.ca/~moyogo/fonts/test/
Denis Moyogo Jaquerye