I don't work at Amazon, but we have a similar system. I've gone all-in on a couple of subordinates saying they deserved a 4/5 for this or that work. And because they were new-hires, I eventually got the grades punched through after a bunch of hemming and hawing.

Also advocated for my own higher-than-average marks on a few occasions. And just arguing the case gave me the grade as often as not. If everyone in the department had been as stubborn and insistent, I don't know that they'd have given the whole floor these grades. But the squeaky wheel...