Probability and statistics are your friend.
What we can NEVER know, is how many miners spent how much time hashing away never finding a hash that solved a block. Can't even reasonably guess ...
We can't know the actual number but we can come up with a reasonable estimate. The chainwork value is a good estimate and a standard deviation value tells us how good that estimate is.
... I could have 1PH sitting here mining to a private solo pool for a year and never found a bock so nobody outside of me would know it existed. And if it DID find a block you would never know if I had a massive farm or 1 USB stick and just got really really lucky.
The chainwork estimate and standard deviation values account for those possibilities.