When running on a treadmill, you will still see some fluctuation of your pace data even though the treadmill is running at a constant pace.
This type of fluctuation is normal. Some of the fluctuation is caused by changes foot movement (our feet don't move at exactly the same horizontal pace each step). Many watches provide the option to average pace for 3s or 10s, which helps make pace look more stable. If you own a Garmin watch, note that Garmin rounds off pace which also stabilizes pace data.
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