I don't think it's crucial to have exact word count. Illustrator notes should be used sparingly. I just quickly count up my illustrator notes then subtract that number from the word count shown. (I use Pages on my Mac, then convert to a .docx.)
But interesting about the "Styles" thing. When I write NF PBs, sometimes I use sidebars in a smaller, italicized font. But to get word count, I've just been selecting each text box, then add them all up. I'll have to look to see if Pages differentiates "styles".