Anyone wants to speculate about the mysteries in the setting?
The numbered books have a six-fingered hand on their cover. That leads to looking at the number of fingers on the characters. As often with cartoons, the character have only four fingers (thumb included). Except... Some characters actually have five fingers. Stan, for example. As well as Soos. Or, outside of the more regular characters, Old Man McGucket or Manley Dan.
So it could be a child-vs.-adult thing, but... Li'l Gideon (a child) has five fingers. And some adults (such as Gossiper journalist Toby Determined).
On the other hand, some bit-characters like Deputy Durland have an inconsistent amount of fingers. [In the same scene, he has five during his dramatic entry
, and four when breaking a lamp