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Because it is not traffic. A page moving from nine to six looks like progress and may deliver nothing if a featured snippet is taking the clicks.
Meanwhile a fall from three to five on a query nobody searches gets attention it does not deserve.
Track clusters, not individual keywords. Individual positions fluctuate for reasons you cannot act on; a cluster's average movement is a signal.
Always show impressions and clicks beside position. A cluster improving while clicks fall is a problem, and position alone reports it as a win.
Track daily, read weekly.
For your clusters, yes — it explains your own movement.
Personalisation and location. Yours is one sample.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.