Date differences when calculating an imported schedule

I have a P6 schedule that was created by someone else and I have imported the schedule.  If I recalculate the schedule immediately after importing (without making any other changes), I get different calculated dates for the same activities.  I've calculated using retained logic and progress override and get the same result, so that does not appear to be the reason for the difference.  The schedule uses global calendars rather than project calendars.  Other than a possible global calendar difference, is there any other reason for the difference in the calculated activity dates?