Most modern electronic recording devices have little change in speed throughout the duration of the recording. Mechanical devices, not so much, what with wow, flutter, and unpredictable speed drift.
It is likely that what is being measured includes the shifts in the 50hz frequency from minute to minute, which would be less masked by a recording that was slightly off frequency.
It is likely that what is being measured includes the shifts in the 50hz frequency from minute to minute, which would be less masked by a recording that was slightly off frequency.