The Network Difficulty is set every 10 blocks by the blockchain to try and reach and average blocktime of 55 seconds, over a span of 12 hours.
If you have 10 slow mined blocks, the blockchain will need to compensate and set the Network Difficulty lower than that, which would lead to 55 seconds blocks.
Because again the Network Hashrate is calculated from the Network Difficulty, it will be lower, than the actual hashrate.
There is no way to determine the actual Network Hashrate, we can only watch and see how fast blocks are mined compared to a difficulty, and extrapolate from that.
Explain by ccodam
