There are no measurements of the network speed, just estimates. The estimates are very noisy, particularly on short time frames.
The correct answer seems to have been missed.
There is no way to know the hashrate of the network. We can only estimate it by looking at block completion time.
IF global network hashing rate was 9,000,000,000,000 hashes/s exactly and never varied the graph would still show variance because it is indirectly estimated hashing power based on block completion time.