20, 40, 80% ownership what does all that mean??? In simple business terms it means how much share of a company you own and consequently how much control you have. If the relationship between shareholders is good as is the case between the share holders of both HM and EY, then who says the HM shareholder has to make all the decisions? Especially if the HM shareholder trusts and believes that EY has more experience/resources to choose a CEO etc..
If me and my mate have a venture and he owns more shares than me but he believes I have more experience choosing the CEO why would he not let me do it? Having majority share gives you the option to block transactions or decisions that you are not in favour with. It doesnt mean you have to make all the decisions.
Anyway its all much more complicated than that. I was just trying to simplify the whole thing for better understanding. I could be wrong though.