Sometimes your pool and mining client report different hashrate values that are denoted with different names: current hashrate, average hashrate, reported hashrate, etc.
Reported hashrate is the value that is displayed on the mining client and also on your minerstat dashboard (as minerstat will show what mining client reports). It is the hashrate you are achieving at the given moment.
Current hashrate is also sometimes called effective hashrate. It is calculated based on the amount of shares you have submitted to the pool, share difficulty, and also luck. That's why current hashrate won't be shown as steady line, but will change - sometimes it will be higher and other times it will be lower.
As the current or effective hashrate can vary through time as it is calculated based on difficulty and the amount of shares being sent, there is also an average hashrate, which tells you the average over a certain period of time. The duration to calculate the average is different from pool to pool - one day, two days, etc.
If your reported hashrate and average hashrate are significantly different, then you might consider making some optimizations. The reasons why this can be is:
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