the rate was correct and was coming from me. it also showed the correct number of shares in the user interface. the statistics delay could be a bit shorter tho. the hashrate dropped after i switched back to single gpu mining, some minutes between the switch nothing was running.
i think the hashrate and number of shares are updated differently. it was still showing the old hash rate but it didn't count new shares anymore because i detached the grid, so there were no shares anymore.