when you submit a share, it has a specific hashrate lower than the total
if you get lucky and submit more shares in a round you will have hashrate > than your actual hashrate

for example my 290x mines at 400KH/s
if it submits shares very rare I may have on the pool a 200KH/s hashrate
if my card submits with a bit of luck more shares I get even 600KH/s

tl;dr your pool hashrate is calculated from accepted shares and not your actual processing power