Project Overall duration Changes when importing.

I have this crazy problem when someone sends me schedule in XER format, after importing; the total project duration is changing, ive tried to make the default global calendar in all options available in the schedule but it did not help.

by the way, the overall duration after importing is the same as the overall project duration (correct), but once i made schedule run it will change.

any ideas about the source of this problem?

thank u all in advance