5k burst on a 4500 psi DOT buddy bottle, why?

Bought a 1.8L buddle bottle new, 4500 psi working pressure, DOT paperwork confirms it but it uses a 5k burst disk. Im sure ill get a blown disk pretty quick if I fill to 4500 so i never do. My bigger bottle uses a 7k disk.

Is this just so it passes DOT hydro tests without damage? I only intend to fill it myself. I have a 6.5k disk I could use but havent.