Yes, the goal for Space Age is 437 the goal for Global is 70. So three ages apart increases the target by over a factor of 6. Why? If anything the battles are far more difficult and time consuming in late game. Even lousy atomic bases still have a lot of ground to cover. My guess is they justify it by saying that atomic and higher bases conceivably could add another 4 generals but get real. Maybe 1 out of 10 bases, if that, ever have their generals active on raid defense. And why doesn't the Pentagon count for this event then?
Like 90% of this game's quirks and issues this breaks down to the fact that no one there who makes such decisions plays this game at a high level, at least not in the sense that a real player would.