im not an programmer but i do use c++ for some projects and i write all base functionality in c++ then use blueprints to manipulate it in-engine (children and such)
example:
i write up a base class for a projectile in c++ and use it as the parent class for a blueprint. then i make a ton of children blueprints based on this (i once had 33 blueprints based on a single custom class made in c++) whats to say thats not what they have hence all the number of blueprints?