It's a balance of maybe 5 things, I dunno.
Some of the playbooks have a more canalized future than others, where it's obvious to me that there are fewer in-playbook directions that a gunlugger will go (for instance) than an angel will.
Some of the playbooks' stat improvements are based on strengthening their strengths - these tend to have fewer - and others are based on strengthening their weaknesses.
Some of the playbooks tend to improve themselves - these tend to have more stat improvements - and others tend to improve their stuff and their endeavors.
For some of the playbooks, essential non-stat improvements crowd out the stat improvements.
And then there's the move thing, where the gunlugger gets Insano like Drano instead of a +1hard improvement. There's a whole different set of concerns there, about what improvement moves I want in the game in what proportions, and should the character be able to start play with the +3, and whatever else.
I'm sure you also noticed that most of the time only the playbook's leading stat can go to +3. A couple of the limited ed playbooks have a strong secondary stat that can go to +3 too.
So: those are the things I'm balancing as I create those lists, and you're right, they come out with tendencies, not with any clear pattern.