The short answer
HealthKit is the system health-data store, not a frame-by-frame pedometer sensor feed. Apple records step-count samples from iPhone and Apple Watch, and HealthKit can notify an app when matching samples are saved or deleted.
Observer queries help with freshness
An HKObserverQuery can monitor the HealthKit store and can be configured for background delivery. When notified, the app can run an appropriate query to calculate the current step total. That is different from assuming a callback for every individual footstep.
Why totals may update in chunks
Apple notes that step-count sample data may be condensed or coalesced. A product should therefore design around accurate HealthKit totals and reasonable freshness rather than claiming a guaranteed instant update for every step.
Primary sources: Apple — observer queries · Apple — step count data type