You'll see that on METAR reports from time to time. They are trying to convey that the RVR is different than the prevailing, and that it is variable. It could be changing often, or it could be that they are saying that the touchdown and mid are different. When on approach, you will never hear that the RVR is variable. They will just read you what it says for touchdown, mid, and rollout. It may be changing often, so you may need to ask again, or stop asking once you hear what you want to. Also, never ask for a RVR report if you are already inside the FAF.