Hey Jap!

A friend of mine in Skypeland from Japan was talking to a Nigerian person who is working in the car industry in Singapore. The Nigerian called my friend in Japan a "Jap" over and over. So my Japanese friend told him that the word "Jap" is derogatory. He asked the Nigerian man, how would you like me to call you a nigger? The Nigerian man said he would not like it, since it is derogatory. Then right after, the Nigerian man went right back to calling the Japanese man a "Jap", over and over and over. I guess there is something that I do not quite understand going on here...