That's more of an issue between you and the ISP than it is a technical issue regarding the conversion between bytes and bits as this particular article addresses.
I, for example, get pretty much exactly what my ISP promises me give or take 3-5%. I live in a relatively low density area where the cable network isn't over saturated (and especially not over saturated with power users). That's not the case in many areas of the country though. Broadband companies frequently over sell their packages and they don't have the infrastructure to back things up.
Historically that wasn't as big of a problem as not many home users were really using their connections to even a fraction of their potential. In the days before YouTube and especially in the days before services like Hulu and Netflix, the only people really chewing up the bandwidth were people downloading (usually illegally) the content that Netflix and the like would later offer. Now that even Joe Average can just pay a few bucks a month and get unlimited TV and movie streaming to his house, the total bandwidth usage in any area has been steadily climbing.
Now all of a sudden it's a real big problem that companies like Cox and Charter have oversold themselves. It isn't people with broadband connections browsing the news and sending email anymore and the rare guy downloading a lot, it's people streaming gigs and gigs of video to their homes.
I wish I had a silver-bullet answer for you, but the reality is that most broadband companies completely dominate the market they're in (so there's little to no competition) and they just don't care. Most of their customers probably don't even notice they're getting half the speed they pay for.