Afghanistan is considered more Central Asian or South Asian

Why?

Because of simple ignorance. Almost no Afghans consider themselves "South Asian" in real life. The truth is that words such as "Middle Eastern," "South Asian," "Central Asian," etc. are just words Western explorers who never set foot in half of these countries assigned to them. They probably just assumed that all of the "stan" countries would be similar and called it a day. Afghanistan is an Eastern Iranic country that sits on the Iranian Plateau. There is nothing South Asian about it at all.