Silver and gold nodes bug?

I think there might be something wrong with the silver and gold nodes.
They are supposed to be a rare spawn spawning instead of tin and iron, but instead it seems they are their own seperate spawn. There are several places in Ashenvale where only silver spawns, and at least one place in Stonetalon where only gold spawns.
I have also never seen a silver or a gold node spawn in place of tin or iron.
I know they changed the spawning mechanics of nodes, but they were pretty vague about what they actually changed. Is this the way it's supposed to be or is it a bug?
They are supposed to be a rare spawn spawning instead of tin and iron, but instead it seems they are their own seperate spawn. There are several places in Ashenvale where only silver spawns, and at least one place in Stonetalon where only gold spawns.
I have also never seen a silver or a gold node spawn in place of tin or iron.
I know they changed the spawning mechanics of nodes, but they were pretty vague about what they actually changed. Is this the way it's supposed to be or is it a bug?