Your hash rate can take an hour or two to level out.
Of course that can be high or low on the pool, but it takes a while to converge.

Also, of course, if you were almost continually mining, i.e. the stop+start was short, the 1hr hash rate will take longer to average out since it's not 'starting from now at zero' but instead averaging out the longer running lower hash rate with the higher new hash rate.