Living in a free market means that nothing in commerce is guaranteed. We have no price controls for most goods and services, so prices tend to fluctuate. The National Automobile Dealers Association has been around since 1917 as an auto dealer lobbying group and, in 1922, started publishing a guide for used car values, per its official website. Kelley Blue Book was started by a Los Angeles car dealer that sprang up from his business buying cars for resale, with the first guide published in 1926, according to KBB.com. The guides have served as valuable tools originally intended for dealers, but both eventually created products for the general consumer market, too.
For around a century, these guides have been invaluable tools used by dealers when assessing trade-in value as well as determining what prices to set for their own inventory. In an age where information was transferred mostly by paper and its movement was slow, these guides were good predictors of value. Slow-moving information is gone. The truth about value is that, no matter what is printed in any book, the true value of anything is only what someone is willing to pay for it. Investopedia notes that while these guides are good, they cannot account for some variables. Some slightly older cars that have drawn a niche fanbase may sell for much more is one example of how the data can be skewed. The best thing to do is study the market and match what you see with the stated values before accepting an offer for your trade.