HubSpot does not move deals between stages on its own unless a workflow or integration does it. A deal stays in its stage until someone changes it. So when a buyer goes quiet, the deal often stays where it was, with the same stage, the same close date and the same probability. Over weeks it drifts away from reality while still counting in the pipeline.
You can see drift in the data HubSpot already keeps. Each deal records the date it entered each stage, its close date, and the dates of its last activity and last modification. Deals that have sat in one stage much longer than your normal cycle, early-stage deals with a close date already in the past, and open deals with no logged calls, emails or meetings for a month are all drifting.
Drift matters because it inflates every pipeline number at once. Weighted pipeline counts the stage probability of deals that are effectively dead. Stage conversion rates look worse than they are, because stuck deals sit in the denominator. And reps lose focus on the deals that can still close. Ancil flags deals that look stuck or past their close date through read-only checks, and any change to them still needs a person to approve it.
The fix is mostly habit. Hold a short weekly review of deals past their expected time in stage, and for each one decide to advance it, update the close date with a reason, or close it lost. Some teams add a workflow that creates a task for the owner when a deal has been in one stage longer than a set number of days.