If a man betrothes a woman and explains that he was mistaken in his thinking about different aspects of her identity, they remain betrothed. She wasn't acting under false pretenses. But not so if they weren't eligible to be betrothed at the time that the man makes conditions on the betrothal, the conditions coming true are not enough - because they weren't eligible to begin with. Plus, a condition to betroth an unborn daughter. Also, the question of that which doesn't yet exist vs. that which does exist, but isn't eligible yet - for example, terumah!