This pool has a high share difficulty, or whatever the best way of putting it is. For me, I get somewhere near 1 accepted share per minute with about 480 kh/s.
As long as your hashrate seems about right and you aren't getting hardware errors (HW on the cgminer screen) you should be OK.
What can be the explanation of getting less shares with higher hashrate?
If you get HW errors (or if your having an unlucky streak).