I don't think they intentionally made an unreachable goal. I imagine that they overestimated the number of active players.
Also, I'm not sure at which point they changed the amount needed for Phase III—whether between testing and live release, or after the Phase II goal wasn't met—but the original goal of 3 billion, I don't think was realistic. If phase I required 400 million, and phase II an additional 1.1 billion, I can understand that increase. People need a bit of time to come up to full production, and get a steady pace of cakes being made. But a further increase to an additional 1.5 billion for Phase III would be a problem. People are probably already producing at their peak during the second week, and there's just no place left to go after that (unless you buy balloons with gems). And what's more, a large portion of the players have finished up all their quests, so you won't have those additional balloons coming in, and some players who have gotten the main quest rewards might even play less and produce fewer balloons. Week 3 will probably see a decrease in balloon production rather than an increase. So it's a good thing that they decreased it from what I guess was the test server amount of 3 billion for the third week.