Based on the responses, it appears that Christians believe that life begins at conception and life can end naturally any time after that per the rules of biology which God created but doesn't intervene. Therefore, a miscarriage is no different than a 2 year old dying of cancer is no different than an 80 year old dying of Alzheimers. That all seems congruent and explains why it should not be considered abortion-like.
So to your point, the question is why did God create the rules so that there are imperfections. Why are there miscarriages? Why are children born with disease? As you've said, you don't have the answer and the Bible doesn't provide guidance on the subject. I do continue to feel that if God were to create the rules, he is responsible for the outcomes whether he intervenes or not. Those unborn babies didn't have to die, and to restate my point in my other post, we don't need to suffer the hardships of a miscarriage to appreciate the life of a baby.