Why do Black people in the US, aka African-Americans tend to score more White-European DNA than those in Europe?

I have noticed that the vast majority of Black people in the US, aka African American on this sub tend score minimum 2-5% European DNA (I think around 12.5 % would be average) on their results while those in Europe (mostly UK, France, Belgium, and Germany) tend to be 100% African.

Is there the reason behind their diffrent result? (No hate or racist I'm just genuinely curious)

Edit: Also I'm not taking about those who are "half-White".