I hear this a lot, but I don't know if it's particularly true. For black immigrants, I find more compelling the notion that their comfort with, and belief in, pursuing higher education isn't rocked so profoundly by white supremacy, as is so for native blacks. They're simply not as exposed to anti-black messaging, from the history of discrimination in America to the demographically disproportionate reporting of crime in American media. Add to that the arguably superior education of black Caribbeans, who are free to support higher standards with the requisite resources and are not stymied by regressive funding policy, and you have a population that is often more prepared for the rigors of higher education.

Which is pretty damning of the way America treats its own people.

Edit: I want to add that this is not simply my own speculation; this is the explanation that I've heard in part from several friends and acquaintances. They're often horrified by the leniency of standards for even high performing American students, and they will often state that they are not "black" in the way that native African Americans are, and are therefore not subject to the same biases and limitations.