Calculated Hashrate shows less Mh/s

Hi
I recently got my rig up and running with 4 sapphire rx 480 cards (2 still on their way). I mine via Claymore on Nanopool. 4 card's reported hashrate is around 98.9 mh/s, BUT the calculated hashrate shows 42.5 mh/s !!! Is this normal ??? How can I increase the calculated hashrate ?
P.S. I am new to mining