I'm pretty sure that is just based on the results of how people voted. Like I could 1-star it on a whim because I had a crappy day. Doesn't mean it was based on how good they're doing with providing advertised speeds.
That said, if you had asked me, say, five years ago, I'd say I never got the speeds I was paying for. I remember upgrading to a higher tier to finally get the speeds I was paying for in the previous one. If enough people had those experiences, I could see ratings dropping.
Of course, I just put up with it. There could have been issues with the modems and signal levels, etc. But this was at five different places. For the last two places over the past four years, I've gotten or exceed the speeds I'm paying for, so they're doing something right.