I think the answer is in the question... the fact that vis IS from the tower reported from objects of a know distance and RVR is at runway threshold at the height of a pilot in the flight deck at touch down.
So if a discrepancy occurs then they use which ever is relevant, RVR is normally only used when vis goes below CAT I, they then use RVR.
Now I'm sure different ATC authorities use it in different ways but this has been my exerience, maybe someone else will quote chapter and verse to help....
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Gravity always wins....