Like many price guides, the operative word is "guide". It would be difficult for it to be otherwise in any context. There are simply too many variables to offer hard and fast pricing. Already suggested above is the quandary of what might be called the ‘sub-models’ and variations thereof. The older Winchesters offer an excellent example of that situation. Then also there is the question of condition. There, the guide valuation is no better than the opinion of the person evaluating the gun. It often requires considerable experience to evaluate condition. Then there is also the projection of making any repairs or adjustments to bring the piece back within desired parameters of functionality/originality. I often disdain the percentage ratings except perhaps on much newer firearms than most collectible Winchesters (or Colts, or Smith & Wessons, etc.).
If there is inherent error within the framework of price guides, perhaps it is greatest concerning true vintage collectible guns. A gun which is five years old simply does not fit in the same condition category with one that is even fifty years old, much less one hundred. By comparison with contemporary models, vintage guns are simply too susceptible to being amateurishly regarded within the same percentage/valuation framework.
For instance, how is "patina" accounted for and graded in the Blue Book? The photo illustrations adjacent to the Blue Book preface tacitly contribute to the suggestion of a singular absolute scale. The percentage evaluations do not suggest any cautionary adjustment based on realities of aging. Perhaps such guides cannot reasonably be expected to account for such without resulting in a gargantuan and unwieldy work. Yet it introduces an important and necessary factor with the singular standard which may tend to be more misleading than not.
Next, how about the "originality’ factor? Really an almost exponential point as other conductibility factors increase. How should a guide deal with that situation? It simply cannot, but at least does not portend to address it as in the ‘condition’ factor.
The result is that such guides are often at best a very generalized rule of thumb for the more collectible weapons. Certainly better than nothing, but perhaps necessarily glossing over almost too many pitfalls for the unwary purchasers and sellers. (Yet as an aside, the sellers nearly always do have a presumptive advantage. They can withhold marketing their wares pending a full investigation. The buyer(s) are often lack this luxury in making a their commitment.)
Next perhaps are regional pricing-valuation differences. One might argue that these have been blurred by the Internet marketplace. But I believe there remain significant differences. I also believe that the prominent Internet gun sale services only superficially affect the actual ‘real world’ market. They represent a small percentage of actual transactions and they are also presumptively distorted by a percentage of both ‘deep pockets’ and ‘imprudent – have to have it’ buyers.
Gun shop pricing also varies across the board, with the trend at least in the last decade to adopt more of the corporate structure pricing. Less based on knowledgeable valuations rather than simply overpricing with a marketing plan of exposing items for sale and passively fostering ‘drift down’ of overpriced items to a ‘floor’ or sale point.
Such is also particularly spurred by exorbitant consignment fees of one third to half asking price; such which tends to prompt unrealistic seller pricing.
Among too many of us, there is also the latent tendency, if unchecked, to adopt “offered prices” as actual resulting market values. We all know better, but the tendency remains.
Exorbitant pricing seems to be highest in the more firearm-regulated states where most, if not all, all transfers are required to be through licensed dealers. Thus offering a sad state sponsored distortion of ‘pricing to market’!
Thus are some of the dynamics of pricing which variables no blue book could possibly be expected to competently account in valuations.
There is an old line attributed to various sources: "Rules are for the guidance of wise men and the blind obedience of fools." So it is with gun pricing.
Yet in all the gun valuation tribulations there is light! Within the joys of the Internet, are the specialized and functional boards of experts willing to share their time and expertise. WinchesterColllector.org is one, where an accumulation of expertise can provide more than simple generalizations. With sufficient quality ‘client’ input, realistic and hard valuations can result. Nothing is perfect, but perhaps forums such as these are about as close as may be obtained!

And so pardon me for exhibiting the truth of accusations concerning my long winded, highly opinionated and tightly vectored… take…