Funny how every time there's a diff jump the hashrate calculation approximation has to be re-explained 
Why do we laugh at new people for asking legitimate questions? Were they supposed to have read the forums a thousand feet deep and figured this out on their own?
They should be laughing at us for having a 50 million dollar "economy" around a website that still doesn't have SSL after all this time, and a program whose user interface is so dinky and devoid of help that it wouldn't even make the Apple App Store as a free app just for its complete lack of intuitiveness alone.
It's funny because that question is answered every time the difficulty changes. Maybe these people could use the search function once in a while, you know...
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i'm not an expert. someone can correct me if I'm wrong but I think when the difficulty changes the hashrate is re-calculated. it is expected that a new block will be discovered ~every 10 mins. At the start of the difficulty level if some blocks are found faster than that then the calculated hashrate is higher than it is in reality. When more blocks are mined at this difficulty level the time to mine them will average out to a more representative period for the real hash rate.
More or less. The network hashrate is established by comparing block resolution speed with difficulty. It's simply an estimate. The block resolution speed is an average taken on whatever period of time. The thing is, when the difficulty jumps up, the average used is still the one established on data from the previous difficulty. Until that sample time spans entirely into blocks found at new difficulty, the estimate will be corrupt.
