This is obviously the truth. But until today, I'd assumed that they used some sort of a point system when designing their cards: for instance, let's say they use some sort of a system where a common card is worth 5 points, an uncommon 6 points, rare 7, legendary 8. Start with a template of a 1/1/4. Subtract 1 point for each turn reduced, each hp added, and each damage added...then you get something basic, like the 0-turn assault units you start off with. Special abilities also take off a number of points--flying might make each hp cost twice as many points, for instance.
However, I realized today that if there is a point system, it is more likely divided amongst the factions themselves than the class of the cards. For instance, take Quad Walker and Dominated Hatchlings: both are 3/5/2, both are uncommon...but the Hatchlings have heal 1.
I guess there's no real moral to this--I just thought it was interesting, and worth pointing out.