The heart rate that you see during the measurement is an estimate of the beats per minute based on the results from the last couple of seconds. Therefore, this is not an exact representation of the actual amount of beats recorded within a minute.
The heartbeat that you see in your measurement result once the measurement has been performed, is the exact number of heartbeats recorded during the measurement.
This explains the possible difference between the heartbeat you see during the measurement and the one in your measurement result.