Two things stand out here. First of all, I had never done a study of this, so I was shocked when I first heard it. You have provided more details here.
What concerns me, is this. We got 400,000 and that’s only about 5%. This means close to ten million were taken out of Africa.
That seems like a very large number for those days.
IIRC, survival rates in the United States were much higher than in South America. We outlawed the import of slaves in 1807 or something - so the slave owners tended to take better care of their “property” here than other places.
The slave traders also lost a lot of slaves in Africa as they were marched to the port, in the port holding camps, on the long ship journey, etc. In S. America the slaves were sent to camps to be “broken” prior to sale (that didn’t happen in N. America for some reason).
“That seems like a very large number for those days.”
Not when you consider it was over the course of about two hundred years.