The elemental schemes reverse each other here. In the Japanese esoteric reckoning the ring finger is water, sui, the second element counted inward from the little finger. In the yogic hasta mudras it is earth, prithvi, and the little finger beside it takes the water instead. Two lists, the same five elements, and this digit and its neighbour swapped between them: the pattern suggests two independent traditions that both wanted the elements ordered along the hand and chose opposite ends to begin from.
Anatomy makes this the least independent digit. Bands of tissue called the juncturae tendinum link the extensor tendons across the back of the hand, and the ring finger is tied by them to its neighbours on both sides, which is why it cannot be raised far while the middle and little fingers are held down. A gesture that isolates the ring finger is therefore hard to make and rare in the repertoires, while gestures that fold it down with the little finger, or extend the two together, are common.
That has consequences for how the finger is used ritually. It appears in the hand seals as part of a pair far more often than alone, and where a tradition wants a single digit to carry a meaning by itself it usually reaches for the index or the little finger. The symbolism assigned to the finger is a choice. The constraint on which gestures are available to be assigned is not.
Sources Mudra: A Study of Symbolic Gestures in Japanese Buddhist SculptureE. Dale Saunders with Asana Pranayama Mudra BandhaSwami Satyananda Saraswati