Block finding is a Poisson distributed random event.
Thus 'luck' affects the hash rate calculation.
Even the 2 weekly diff change is not a highly accurate estimate of the network hash rate.
No one knows the Bitcoin network hash rate.
They can only estimate it from the rate at which blocks are found.
That estimate is inaccurate.
Using 250 blocks to estimate the hash rate has a 42% chance of being more than 5% inaccurate
https://bitcointalksearch.org/topic/--5307087
.. at the top end no one would buy bitcoin above $72k supportively. because they can just mine the heck out of it cheaper supportively
...