Yes, it's a "definition" question/problem: a SR is the exact place where you store your virtual disk images. On NFS, it's a folder named after the UUID of the SR. As Dan said, if you connect to the same NFS folder, it will create a new folder for each new SR inside. So even if it's the same physical device/NFS share, there's many SRs on it.
For a block based SR, it's different: if you pass a LUN (in ISCSI for example), it will attach it and warn you: either you want to re-attach the previously existing SR (UUID etc) either you erase it completely to start from scratch. There's no "folders" on a block space like a LUN, so it's different.