Amazon: How do they track you when voting?

It is a strange thing.

I have like 10 different accounts with Amazon. All accounts can write product reviews fine but when I vote on a review:
"Was this review helpful? Yes/No"

Only one of my vote counts. It doesn't matter how many times I vote with all my other accounts.

I've tried using different IP address (hide-my-ip), and incognito chrome browser.

Amazon seems to know that I am voting twice and they are not counting my vote.

Any advice?

Thanks! :)