I think I have a partial explanation of the FHWA's total for I-74 mileage in NC. If you take the three sections currently signed as I-74: From the VA line to US 52, from Exit 65 in High Point to I-73 in Randleman, and from Alt 74/Bus 74 east of Maxton to NC 41, you get a total of around 57 miles, not the 53.4 listed. However, my 57 includes about 4.5 miles where it is signed with I-77, take that away, as they do (partly) with I-73, and you get around 53. This doesn't explain why they count only 16.3 miles for shared mileage with I-73. The total should be around 55 miles. By looking at the mileage of the different segments signed I-73/I-74 over the years, you only get around 16 miles if you include the original segment that ran from US 220 Exit 51 to 39 (12 miles) and the last segment signed, from Exit 76 to 80, the 4 miles north of Asheboro to where the two routes split.
So the log entry for I-74 should look something like this (counting the Forsyth County section that is approved, but not signed):
I-74 NC 122 miles - 62.5 (I-74 only) (w/ I-73 55 miles) (w/ I-77 4.5 miles)