I noticed that the time in zones, using an identical FTP and zone definitions, is reported slightly differently between Strava & Intervals. For example, Strava might show 90s in Z7 and 500s in Z6, whereas Intervals might show 60s in Z7 and 530s in Z6. I assume this is because of the precision of the FIT file used during interpretation - should I consider Intervals or Strava to be more “precise”? Thanks!
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Neither, those are rounding differences.
A % of FTP is seldom an integer, some platforms ´round´, others ignore decimals.
People who consistently target the high limit of a prescribed range, will have bigger differences. Don´t do that! A range is a range, stay within the prescribed range.
And the higher the zone, the less important those numbers are. Outcome of Z6 or Z7 training is more dependent on total time above Z5 (VO2) then on actual Z6 or Z7 time.
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I don’t know how Strava calculates time in power zones. Intervals.icu uses a 5s moving average of power. Otherwise you end up with a lot of spurious time in high zones just from accelerating away from traffic lights and so on.