As far as story goes, I think Plasma Captain's suggestion is probably the most intuitive (and interesting.)

It certainly leaves the most room for flexibility, the idea that the spaceships are all built by, or using the technology of, societies and worlds all over the universe that the robots have encountered and, having limited contact with one another, built along very separate lines. I kinda like the idea that the robots don't have much understanding of technology of their own, but that they're really good at assimilating it from others, which would explain why their ships would have nothing in common in terms of design, aesthetic or materials.
So maybe these assimilationist robots have been built, perhaps by
Vitacorp like Gridlock suggested as a misguided universe-domination scheme (I already have a logo suggestion designed in this case!), perhaps by some mad scientist as a bizarre experiment, by some evil alien race etc. etc. who have been sent all over the universe to assimilate the cutting-edge technologies of every society they encounter, which the eventual objective of forming the ultimate high-tech robot empire as they conquer planet after planet, building galaxywide fiefdoms and eventually converging on each other with all the best technologies that the universe has to offer! But Keen got wind of this evil plan early on, so no real convergence has happened yet and the robot battleships are all built on very different levels and types of technology.
Or something. Just keeping the spitball rolling (though spitballs don't really roll.)