I notice that sometimes the difficulty(a number from 800 to 3500) cannot reflect the true difficulty of a problem,

like: CF1603F

Only three strong programmers passed the problem: Two LGMs and rainboy, but its difficulty is only 2700.

I guess this low number may be because rainboy's rating (about 2000 at that time) is far below his real level.

So, I would like to know how codeforces determine the difficulty of the problems?