Page loads in 1,93 s and PageSpeed Insights says LCP is 2,9 s

This is for mobile. Anyone knows how does that #$%#@ tool from google work? After optimizing the website it tells me the LCP is shown 1 second after page is fully loaded.

GTmetrix shows correct values. Almost the same results I get in dev tools.

Question is: how does PSI work? Are the results cached? Is it an average result for 28-day collection period and I just have to wait?

Real LCP for mobile is 1,6-1,7 s. I don't think I can get it any lower, currently pages score 93-96 for mobile and 100 for dekstop. Should I be worried about it?