ACS Publications editorial staff format the PDF and HTML article versions differently, most notably with the numbering style of the in-text citations and reference lists. This is to optimize readability for the different formats, as well as technical considerations like navigating within an article on their website and linking out from the articles. These variations may be hard to replicate exactly without the benefit of editorial support.
ACS Publications does not require that you use ACS this citation style. While they do refer to the ACS Style Quick Guide in the submission instructions, authors are free to use any citation style when submitting their article and book chapter manuscripts per the "Fast Format" guidelines, as long as they include the article titles. Their technical staff will reformat the accepted manuscript (including references) before publication.
About citation styles (in general)
Reference formatting is fairly clear with journal articles, patents, books and chapters. but gets increasingly fuzzy as you move into other online sources. Especially for sources where users upload their own content, and name or describe their content in ways that don't allow for easy, standardized citing.
There may be multiple ways to cite a source, based on 1) where you find it, 2) how it's described, 3) how you decide what kind of source this is, and 4) who is identified as the creator.
Reference managers and generators will work best with commonly cited sources (or at least easier to fix), and less so as you move away from those common sources. Will involve more work, whether editing the output style or manually editing those specific references. In the end, it may be easier to correct the reference in the final paper then to spend the time to get the reference to format correctly in your reference manager.
