In the Mythos and History of the cults, you can assume that those stories are associated with the cults wherever they are found. That also means that there are countless local stories told by that community but unknown elsewhere. What is presented in the Cults Books is only what is universally acknowledged (or if it is not, it is clearly expressed as a local variant).
Some cults like Orlanth have a very well-defined mythology known from Prax to Fronela. This is the product of centuries of development, countless priests and lords, and world-shattering heroquests. There’s also thousands of local stories, that complements this well-defined mythology.
Other cults have only a few universal stories. For example, Yinkin’s priests seem to make up new stories every time they meet. But as there are likely less than a dozen priests in all of Dragon Pass, that doesn’t end up being that much.
You as players and gamemasters are invited to add your own stories to what is universally known.
So with the new books there is both a canonical core mythology and room to add to it. With the smaller cults there is usually far less canonical core myths and with the larger cults there is much more but that is in the nature of such things.