Former manager at big tech here and this is my experience, not sure it really translates to other companies.
Usually higher ups constrain line managers of ICs with limited budget (most of the time you don't know what it even is). Moreover, the process is inherently flawed: I may help folks find opportunities for impact and it may even be great impact in my eyes, but eventually higher ups want to just promote the really really good people, which will inevitably result in comparing everyone with everyone. Most of the time I was not in these conversations.
It's unfair because most people work against some goal that's agreed upon, and later that year the criteria changes.
This is to say, no one is really in full control in some companies. In 12 years, I have yet to find a process and surrounding environment that promotes fairness. When there's limited money and a true meritocracy is enforced, there's loss of control. Yet, managers should own the message of why you don't get promoted, even though you should in their eyes and there's no clear reason for the outcome (EDIT: other than "someone, somewhere was better than you" in some way which is very disempowering).