Just registered here to say I'm experiencing the same issue as well.
I've been mining for 2 day continuously, claymore's miner reports almost constant hashrate of ~700 MH/s yet on nanopool it shows ~600 on 6-hour and even 24-hour averages.
So, I stopped mining completely for 1 hour then restarted for 1 hour then stopped again and counted the number of (accepted) shares.
On nanopool the sum was around 18% lower than what shows up on the miner.
WTF is going on ?

(BTW, I'm also getting around 1% rejected shares, but that's clearly not the issue here)