I believe a large number of entries are misclassified as RSV instead of RSVT.
RSV reservoir(s) an artificial pond or lake
RSVT water tank a contained pool or tank of water at, below, or above ground level
Code:
SELECT country, count(1)
FROM geoname
WHERE fcode = 'RSV' AND name LIKE '% Tank'
GROUP BY country
'GY', 1
'IN', 55
'KE', 5
'LK', 87
'MM', 5
'US', 15623