That comes about because of the source materials. The pastiche novels have it one way, the comics have it another. The maps the artists use to base their own maps often have their own errors which get continued on.
A good example of this is, as I have said before, the mountains around Khauran. Someone made a map with the Kezankians connecting with the Pyrrhenian mountains, which is an error and not consistent with Howard's description. Later mappers, instead of doing their own research, used that map to create their map - thus the error continues. Even though the AE and ROK map corrects this, the map in the original core rules and the map used in the new novels show Khauran to be a mountainous nation.
There are a lot of errors in all the Conan maps (from the Gnome Press maps, to the Ace/Lancer maps to the comic book maps and onward) because of these reasons. The Black Kingdoms and Stygia maps are horribly flawed, as is Hyrkania and almost everything to the east of the Vilayet. (The best map of Stygia
was produced by Dale Rippke).
Another example is the Westermarck. Almost all the maps incorrectly place it between the Thunder and Black Rivers. I didn't even notice this was wrong until I was doing research for ROK, and I had to lobby very hard to get the map changed and have the Westermarck placed in its correct location according to Howard's texts. Another difference between the ROK/AE map and the original in the core book is that location change. But since most Hyborian cartographers use older maps to base their maps on, few of these maps have the Westermarck correctly placed. Most cartographers are not Howard scholars - they presume earlier cartogrophers had it right.
The Yellow River is not from a Howard source, so it is hard to decide where it really lies when all the pastiches treat it differently. One storyline in the Savage Sword of Conan actually had Brythunia and Nemedia as coastal nations! Obviously, when authors cannot be bothered to research the source materials, why should the map-makers do any different. No matter what, it is impossible to create a map that justifies all the source material due to such conflicts.
So, while I believe it is possible to have a map consistent with what Robert E. Howard wrote, once pastiche elements are included (such as Yellow River and the capital of Brythunia), the maps are doomed to be inaccurate and confused. However, to produce such a map, the cartographer needs to let go of all previous maps except those hand-drawn sketches produced by Howard - and go from there according to the various texts.