61% of white South Africans are of Dutch Afrikaaner decent; if you are the rare type that values facts or statistics. Just like most Americans misunderstand the majority of people in Northern Ireland are English. That is why they do not want to join the Republic of Ireland. But the minority wanting to join Ireland is a large and vocal one.
In the case of South Africa, the reason most people consider it British is because of the success of the British military @100 years ago, not the ancestry of those Caucasians in the nation.