Corporate HR, based on actual thinking unlike the Twitter author who seems based on ego. The reason for this is there's a lot embedded about what 'A', 'C', 'E' mean, and that's different for different people. Using specific terminology (somewhat) mitigates this.

'Average' is also 'Effective'. If the organisation's hiring most people at the right level, that's the way it should be, right?

[An organisational pat on the back that its employees are effective. And then whole business lines get cut, because they're not effective. But the employees are, correct? Blinkered into effectively obeying. That's different from the OP.]

Consistently getting 'exceptional' or 'outstanding' is a problem for your manager because it means they've hired you at the wrong level. Whether this is a problem for you depends on your manager.

It's not all in a name, it's how you think about it. That the author of the post deflates 'Effective' to 'C', which means overall able but lacking in in some areas, identifies something's off. Enough to make me run a mile from any association with this company.

Further, something wrong with the employee conversation. Given a job specification and reasonable leadership communication, having someone surprised means they've been working with the wrong goals, and that's leadership's problem. It happens, I've been there, and then you reverse it, however Tweeting it as a bragging point is lost on me as leadership communication.