Under the hood forum groups are indeed tied to posting although forum members can belong to more than one group.
For instance, 'A:M User' is the catch all group everyone belongs to in order to make sure access to all the various corners of the forum is granted.
Titles such as 'Craftsman' and 'Mentor' were originally designed to compliment the general idea related by A:M software which at various times went by the names 'Apprentice', 'Journeyman', 'Master'.
A thought behind the titles was to try to tailor some of the 'A:M Experience' toward the interests of each user.
The general idea being that while everyone is an A:M User some have demonstrated their craft in ways that separate them from others.
Nothing really became of the latter titles with regard to the forum.
As most forums work, the auto increment feature of a forum based on number of posts is designed to reward a member for participation.
This generally is inadequate to reflect reality as posting a lot don't necessarily equate to mastery... sometimes it just suggests verbosity.
Specific groupings such as 'Film' or 'Hash Fellow' step outside of the primary categories to designate specific membership which may or may not also allow access to different forums.
Care must be taken to keep everyone on a level playing field and these special categories have backfired in the past when members of a special group such as 'Film' had their 'A:M User' group removed only to discover they no longer had basic access to the forum.
I'd have to check but groups such as 'Craftsman/Mentor' should be the same as 'A:M User' permission-wise but as you suspect longevity and posting in the forum have elevated them from one group title to another.
For anyone familiar with general networking protocols most forums operate the same way with permissions.
This is can be illustrated by members that belong to no group... who have read access to most of the forum but cannot post.
The 'validating' group is what everyone belongs to immediately after signing up which gives them just enough access to hang themselves if they decide to spam the forum with ads, inappropriate posts and trolling.
At one point there was a little effort made to move toward a system where members of the forum select their own groups.
Evidence of that can be found if you create a new login where a few questions are asked related self assessment of interests and skill levels.
That direction tends to be pretty high maintenance... someone has to maintain and update those... so that also didn't go very far.
I'm not sure if that answers your question but that's the general background.
Additionally The ability to edit the profile title which in your case you've changed to 'Rigging Masochist' is granted after a specific number of posts.
This gives the forum member some control on how they want to be perceived by others.