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Frequency metrics question

Frequency metrics question

The UK 50Hz grid frequency is manipulated to signal generators and heavy consumers that more/less generation/load reduction should be implemented.
The Tesla GW reports grid frequency so I thought I would capture this and see if I could see the process (Enhanced Frequency Response) in action as the thresholds are not fixed but seem to generally be set about 48.7Hz and 50.3Hz.

There are two readings: site.frequency and battery.frequency. My setup does not disconnect from the grid so I expected the two readings to be the same but they often disagree by up to 0.075 Hz. Apart from a sampling error is there a reason why these figures may not agree? or am I missing some basic AC principle here?