I'm playing with mining ETH and the client written in C.  The output shows I'm mining at 10.3 Mh/s and I contribute to  the Suprnova pool.  The pool says I mine at 16Mh/s and goes up to 27Mh/s.  I can't figure out if Suprnova is wrong (when I mined other currencies in the past it seemed accurate) or if the client is under reporting.  How do you guys determine your real hash speed?