It's worth considering that FHWA's data may be wrong, due to the person who entered it being confused. AASHTO does not appear to have the FHWA definition from 1972, but everything from AASHTO and PA shows one continuous route:
Conversely I-676 would be relocated along the present I-76 via the "Vine Street Expressway" across the Ben Franklin Bridge and junctioned at the present interchange in New Jersey.
On the other hand, the March 2001 Route Log and Finder List has 2.15 miles for I-676 in PA, while the March 1992 length is 2.59. So did something change in the 90s? Anyone want to ask FHWA? Not that this would necessarily change how we map it; see the cases of I-80 CA, I-495 NY, I-695 MD, and I-895 MD.