Why do the expected, required and promise dates differ between order, production and purchasing screens?

As a daily user I have found in the past that when reviewing the commitment schedule under the Item Master Inquiry, the dates between order and production screens were inconsistent. This caused an issue with having material ordered and delivered in time to build. Just because the screens may say expected date, required date, promise date, it seems like they are not consistent across all screens. I am unsure as to what date is pulled when viewing the commitment schedule and would like some concrete insight on that!