Here we are looking at something on top, which might not be annual, is certainly not guaranteed, and is in effect a gift from the company or in this case a single shareholder, who has made more money than can ever be spent. Its a gift which is not related to performance or contribution at all. Hence why it should be flat rate.
It is simply a case of a kind shareholder helping out the people who work at her company, unprompted, unprecidented, indiscriminatory of anything. A gift to people she works with every day, from someone who has that much it simply wont be missed.