When you have a task that has a 3 day duration assigned to a person, and then look at that person's allocation in their user profile, you'll notice that it spans 4 days.
The user's timezone and schedule's timezone don't match.
The system default schedule is Central time.
The User's schedule is set to Pacific time.
Schedule Details shows that even though the name is Pacific time, the selected timezone is Central time
Issue: Looking at a task's start time of 8am, the user would expect to have 8 hours to work on the task.
Problem: User's time may be 8am, but because their schedule is Central, the back end thinks you mean 8am Central, not 8am Pacific, and thinks the user only has 6 hours to work on the task that day.
This then means that in order to fulfill the 3 day planned duration, it will allocate 6 hours on day 1, 8 hours on day 2, 8 hours on day 3, and spill over 2 hours into day 4.
This is based on settings found in Setup -> Project Preferences -> Projects -> Timelines -> Typical Hours per Work Day (default is 8).
Set the schedule's timezone to the appropriate timezone (Pacific for Pacific timezones.)
Make sure the user's timezone and schedule match (Both set to Pacific for example.)