I'm losing you ...

Based on your # of women: 151,600,000

Say 2.5 pregnancies to term/woman:
379,000,000 pregnancies (likely high)

In the US there are 11 fraternal twins/ 1000 births:
4,169,000 fraternal twins

Taking your 1/400 mixed fathers stat:
10,423 fraternal twins with mixed fathers

Assuming even distribution, that's less than 150/year over 70 years of life expectation.

But, too me at least, this is still astounding!!