I have noticed that since around the time the total average hash rate changed to the last 60 minutes, the shares per worker display has become erratic.  It used to show any change at each page refresh, and the number of shares always increased steadily until a round ended; now the number updates only after an interval (I have not yet determined how long) and the number of shares sometimes falls:  for instance, just now when I refreshed the page one of my workers fell from 1933 to 1931 shares.  Some minutes earlier it had dropped from 2053 (and now I see it has fallen again to 1905).  The round and block number (299880) had not changed, the Mhash/s for each worker is within the expected range, and the time since the last share is usually zero.  The estimated reward also fluctuates wildly: while I have been writing this it has varied between .00002876 and .00007421.

Could this be anything to do with the high variance in payments per round that I am experiencing?  Has anyone else seen similar behaviour?