Anon, I dont quite know the answer. I am guessing that since men tend to die younger than women, there are a lot of older widows that were taken in as a chesed.
Here is a thought I just had though – men generally marry women younger than them (my great grandparents were 20 years apart approximately). Each generation produces more than the previous one. So, if a man marries women from the “next generation” (since every 20 years is a generation), there are extra women for him. I think that works out mathematically, as long as the population growth is increasing.