This problem may arise because you assigned different calendar working hours to project than the scheduling working hours for the primavera software.
For Example : Activity A has 4 days with calendar 10 days —–> this equal 40 hours for the activity
although the default for the primavera from the admin reference says that the day contains 8 hours
so in the start and finish date you will see that the activity
Start : Day(1)
Finish : Day(5)
Duration : 5 days , instead of (4) days
you can change this form the admin preference as shown in the attached photo , and this will match the dates with the original duration.
I hope the answer is clear for you and if you have any comments or more clarification , please don’t hesitate to reply.
Thank you ,