Your ant miner includes rejected shares and hardware errors.  Additionally, the pool dashboard is an estimate based on accepted shares, while your local miner is actual hashing speed.  The average on your Dashboard will always be +/- 5 to 10% of your actual hash rate (even further off if you have HW errors and/or lots of invalids).  As for "always when the pool luck is extended bad run", this is called selective bias.  You see the same fluctuation regardless of pool luck, but you're trying to associate completely independent events.  tl;dr:  Your brain lies to you.