Not surprising when you don’t mount those folders into the nginx container, only the wordpress container. Mount is not copying files. What happened is probably that you used a common volume mounted to both containers and tried to bind mount two more folders onto the exitsing volume, which means that empty folders have to be created as mount points which is indeed a change on the volume so you see it in the other container, but the mount happens only in the wordpress container. Each container has its own mount namespace. That is why containers can have their own root filesystem. You have to mount the folders to the nginx container as well or configure the nginx proxy to send all requests to the wordpress container where you write a php script or module that handles these requests even when there is no PHP there to handle and the php script can then handle static contents and use something like readfile to send the content to the client immediately.
But the easiest way is mounting the volumes in both containers so nginx knows about the files.