Originally Posted by
LoganFlyer
I don't know the details, but it's not unique to the 763s. One reason that DL has multiple 752 configurations is that they're different sizes. If the original buyer of the plane wanted a slightly different size and was willing to pay the money for it, Boeing wasn't going to say no!
This has to do with exit door arrangements, which results in differing amounts of usable interior floor space for seats. Both the 763 and 752 have more than one exit door layout option. If the dimensions of the airframe were different, it would seem possible to affect flight characteristics such that new model designations and certifications would be needed through regulatory agencies.
N394DL (and some other now-retired examples) has a second set of full-size doors in front of the wings and one set of over-wing exits. The other 763 aircraft in Delta's fleet have only one set of full-size doors ahead of the wings and two sets of over-wing exits. This changes the number and locations of seats that can be installed while allowing access to all the doors.
In similar fashion, some 752 aircraft have mid-size doors immediately behind the wings, full-size doors at the very back, and no over-wing exits, while others have two sets of over-wing exits and only one set of full-size doors at the very back of the cabin.