Why would my due dates be different on two projects with the same start date, task constraints, and duration type?

[ElaineHe]

‚Check the Project Owners and the project settings to see if you have incorporated Weekends / Holidays in the set up.

[Femke_Crijns]

It is possible that an assignee is on holiday, as registered in the Workfront calendar, and that therefore the planned end date of the task has been pushed further.

[denise_moore_ameritas]

The above two comments are true and also IF the planned hours and/or planned start TIME isn't the same on all on both project or the tasks within them, it could throw off the planned completion date.