No need for Matusi but commonsense is needed. This school happen to be owned by a rich person from central and is in Muranga. This is an expensive school and other than the normal marketing at the national level, there is a likely hood that one of the most effective marketing strategies is through referrals.
This would means, friends of the owner and their friends, relatives and their relatives and friends, employees at equity and employees of other of his other companies and their friends and realtives are more likely to know about the school and believe in the school more than those who see adverts in the newspaper.
Whether in primary or secondary, A person living in Nairobi but with roots in central will most likely take their kids to school in central if not nairobi. The same things appear to be true to those who come from other regions.
You expect a private school in Luo Nyanza to have more Luos. This is the same case with other regions. The same way you expect a mortuary in Bondo to have more Luos. How come no one questions that some international schools have almost all wazungus, others have almost all wahindis. And all this may have nothing to do with people being locked out.
And I think in Kenya, we tend to call a school international not because of its composition but because it offers other curriculum other than KCSE...read the British Curriculum