u only think that because blacks tend to live in specific states/cities/regions instead of evenly dispersed throughout the country. most live in the south where they make up 25-50% of their cities, so the idea we are only 13% often confuses us cuz we look around and see so many black faces. however, there are large areas of the country with no blacks or a minimal amount of blacks.
as for the arguments about north africans and mexicans -
north africans are a like a third of a percentage in usa. even if they were all counted as black (i don't think all are) that would not make us 20-25%.
how mexicans are counted matters to whites but does not directly affect black %. if mexicans are counted as mixed our number would still be 13% because total usa population does not change. white % would go down.