Looks like in windows QFontDialog shows all (regular,bold,italic,bold italic) as a default. In Mac QFontDialog (also not native) shows only regular if there's no other styles available to font. In Windows it's not possible to check font style because QFontDatabase.bold(family,"bold") returns always true.

Easy to test by launching QFileDialog and comparing selected font available styles in fontdialog to windows control panel fonts list which tells real available font styles.