How long of a period are you taking to get shares/hour? It can vary.
I used 2 different pools with LONG rounds and monitored each for 3 hours on 2 seperate occasions/rounds.
**I should clarify though**
I run 3 cards (all 6870's) in the test machine in question, with the lowest card starting approx 260+ and the top card of the 3 doing 290+ Shares/hour during the testing/monitoring, using the most basic OpenCL setup in Guiminer with "-v -w128 -f1".
The same card was not always the highest producer, the 3 cards all maintained 260+ and each would peak at over 290/hour for periods of 30mins or more before leveling off to 280-ish.
Prior to this, clocking the cards as high as I could get them stable, producing nearly 315Mhash/s each.....they would never peak over 250 shares/hour.