IMO, any book or site I have seen is probably rating a rapid or section about in the middle of the possible range. There are so many variables, a person's experience on any given day could easilly be a solid grade higher or lower, depending on conditions. A lot of it also has to do with why a rapid or section gets the rating it does. Some IV's may get their rating based more on how technical they are vs another getting a IV based on consequences vs. another based on size, etc. Depending on what a person's strengths and weaknesses are, that person may have a completely different outlook on the rapid.
Just look at several sources to see what I mean. One book will call a rapid a IV, another will call the wmae rapid a II and yet another will call it a III+. The more detailed the source, the more credibility I tend to give it. For example, the source that give multiple ratings depending on water level are more useful and more credible to me personally. Take the Arkansas River Guide for example. It has as many as 4 ratings for some rapids, and descriptions to go with them as to why. EddyFlower has a single rating and description (nothing against EddyFlower, it's a great resource for many reasons) for a given rapid or section.
Individuals you meet on the river are certainly sources too, but just because they say it's a IV, doesn't mean that's the general consensus. Look around this site. You'll find a lot of disagreement on ratings on various rapids here too. I think if you take a few different source and get a general feel for a rating, then go out expecting it to be anywhere from a -1 ot a +1 from the general rating, you'll be in good shape.
This is all my own personal theory of course, based on my personal experiences....