It all depends on how they calculate the total hashrate of the network... As far as I know, there isn't one accurate method, as that would require knowing API address of all mining pools in the world and fetching the individual statistics at the exact same moment.

It is also possible that the drop happened due to blocḱ(s) where the actual solving difficulty was a lot less than the expected difficulty. This is because "total" hash rate is essentially block difficulty divided by expected solving time, not the actual solving time.