I think we can all agree that there should be a relation between the badges to indicate their significance, one such relation is the color
(gold > silver > bronze) that is already implemented.
It's only natural for the shape
to also constitute an indication of each badge's significance.
However the current badge shapes (lozenge > circle > triangle) do not follow any logical pattern.
I think it would make a lot more sense to associate the significance of a badge to its complexity in terms of how many edges the shape has.
Which results in the following (lozenge > triangle > circle) instead:
:)
Once you've been around for a bit, none of this really matters. I'd imagine the circle is intentionally placed in the middle to deter the possibility of a "quick glance" and mixing up the hard edged shapes. With the circle dividing the edged shapes, it's pretty clear even if it's not intuitive to everyone.