It's the only logical explanation. I mean, cloning isn't exactly high-tech for sci-fi anymore. At this point, it's now high-tech contemporary, which makes it low-tech sci-fi. If humans were to suddenly find themselves in a galactic colonization race where more people were suddenly needed in vast quantities, people would very much take to cloning technology. It's a lot less unpleasant than trying to put them out through your crotch. A growth rate still exists because society can only integrate new clones so fast. This is also why a colony with 15 million people living there can create a colony ship that has 50M more people in it somehow: They clone up some people, pack them into the colony ship, and send them off. Since these extra people are being sent offworld, there's no concern that they are going to negatively impact local society, what with being unemployed bums and all.
the problem with that is that those 50M are breaking the "society can only integrate them so fast" rule. Who raises those 50M clones?
I am fine with cloning accelerated breeding, but those people still need to be educated, taught how to speak, read, write, social mores, and sciences.
Have you heard that term "Magic A is Magic A"? it basically means that in a work of fiction you need to be self consistent, it is ok to make unusual rules, you must follow them though. If 15M people can clone 50M people to build a colony ship in 3 months, then they should be able to do the same in order to fill their planet's capacity. And while cloning people for a ship, they cannot clone people for the planet, so growth ceases while building colony ships... a more sensible approach, and an internally self consistent one, is to explain that the ridiculously high growth rate does involve cloning, and when you build a ship simply remove said people from the planet. BTW. if that was the rules of the game, I would be ok with it... i have nothing against a colony going from 0 to max population in under a year due to massive scale cloning.
there is also the balance issue, because they are created when you build the ship, you are essentially using money to buy population, and population generates money. So the more you buy, the richer you get. thus colony spamming is overly valuable... note that with 1.0.5 you can still colony spam, its not as ridiculous as before, but it is still pretty useful. I find it nicely balanced now at 1.0.5. I can still colony spam, and its still very powerful, but not as crazy powerful as it was. And self consistency is nice.
I do not have a superman complex; for I am God, not Superman.