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

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

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.

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.