Why do black ppl think there mixed even when there not!?

ok well ive noticed this with lots of blacks and they all think there mixed.not one of the ones ive ever talked to said there were fully black they always like to bring up "there grandma" is mixed. i mean seriously anything past the second generation doesnt count for you anymore. i mean i never say im mixed, even though i have light skin and puffy brown hair, heck to be honest my parents are straight from the congo and im not ashamed to say that. but some blacks even laugh at africans and made fun of them, when there so ignorant to face the fact that there great grandma and grand dadies were brought here from a slave ship. i came here by plane, so atleast i dont make up where im from. like what the heck, why do all of them like to say im mixed with "puerto rican, white, asian, arab especially indian" and if they do talk about africa theyll say egyptian. and what the heck is going on with everyone being "cherokee" like seriously theres a bunch of other native tribes like the "mohawks" but they had to bring up them cherokee's. like comeon we can tell if your a mut or not the whole black race cant be mixed!