I recently obtained a copy of AASHTO's 1989 US route log. The pdf version included a table of contents page, along with a listing of mileages for each US route. But the verbiage on the TOC expressly noted that the mileages listed there included the bannered routes associated with each mainline route:
(https://i.imgur.com/G82pUOC.png)
So in other words, the mileage totals listed on that page were never intended to reflect the actual terminus-to-terminus mileage of the mainline US routes. Yet unfortunately in the 30+ years since, that's exactly how those figures have been misconstrued by all kinds of people, from the authors of FHWA articles (https://www.fhwa.dot.gov/highwayhistory/history_misc.cfm), to the editors of Wikipedia pages, to state DOTs that have manufactured signs like this one (https://www.usends.com/uploads/7/5/0/3/75032313/020-newport-min_orig.jpg).
By examining the detail pages of each US route, one can add up the state subtotals to calculate AASTHO's actual end-to-end mileages (as of 1989). For any routes that had associated bannered routes, those end-to-end mileages will be less than the numbers listed on the TOC page. As one example: US 1 on the TOC page was listed as 2593 miles. But the detail page shows that figure included 216 miles worth of bannered routes, and that the total mileage for mainline US 1 was actually only 2377 miles.
(Credit goes to Mapmikey for discovering that bannered routes were included in the mileage totals, but until now I did not realize that AASHTO stated that from the outset.)