But then you also have to account for immigration of blacks to the US, the fact that maybe in 1850 a family may have easily have 7 or 8 children where as in 1950 maybe 3, and that the rate of importation of slaves probably wasn't constant and changed from year to year. Also like already mentioned probably plenty of slaves brought over and not recorded as slave importation still occurred when it was officially illegal so a lot of slaves probably don't have records from shipping.If deaths were involved it would be less especially over months in the oceans. The only answer that makes sense is black folks were already here before settlements.
Point is, there are too many variables to consider but when using a rough estimate, the rate found is not absurd to say the least and in fact, incredibly normal.