The smallest digit carries the two schemes' opposite ends. Japanese esoteric Buddhism counts the five elements inward from here and begins with earth, chi, the densest and most solid of them. The yogic hasta mudras give the little finger water, jala, and put earth on the ring finger instead. Where the two lists disagree they disagree by a swap rather than a shuffle, and this finger and its neighbour are the pair that were exchanged.
Physically the little finger is more independent than its size suggests. It has an extensor muscle of its own, the extensor digiti minimi, which the index also has and which the middle and ring fingers do not, so it can be lifted clear while the rest of the hand stays closed. That is part of why a raised little finger reads as deliberate in so many settings, and why hand seals needing one digit extended from a closed fist can use it.
Gestures involving it are unusually varied across cultures and unusually specific, from oaths sealed by hooking little fingers together to the severing of a joint as an admission of fault in some criminal societies. The atlas records what a particular tradition assigns to the finger, not a single meaning running through all of them. There is no such meaning, and a scheme that produced one would be describing itself rather than the practices.
Sources Mudra: A Study of Symbolic Gestures in Japanese Buddhist SculptureE. Dale Saunders with Asana Pranayama Mudra BandhaSwami Satyananda Saraswati