How the FCP - first contentfull paint - is measured by the browsers?

Question

I did some performance analyze of my application. So there is lighthouse report on the browsers, and when i generate report from them i get 21 seconds seconds on my first contentfull paint result.

FCP is basically the time until the first element is rendered on the browser.

But when i measure the time my self, so i reload the page i see that 5 seconds are needed until my page is fully loaded. That means that if 5 seconds is needed for my whole page to be rendered on the screen than the FCP should be less than 5 seconds where the first element is taken into consideration.

So i can't understand how Chrome or Microsft Edge are generating this report where it says that 21.5s are needed for FCP?