Why is it difficult to obtain accurate end-to-end mileage for many US highways?
This comes as a surprise to some, but US highways are actually not federal highways... at least not in the sense that they are owned and maintained by the federal government. Rather, the US routes are actually just state highways... although they are "special" in the sense that at some point they were granted permission by AASHO (later AASHTO) to be signposted with a US route shield.
Originally, the primary purpose of the US highway system was to identify the most heavily-traveled routes and to aid travelers on those roads by providing inter-state numbering continuity. But since the federal government does not maintain these roads, there is no federal authority that has a reason to keep accurate route logs and mileages. The closest such entity would be AASHTO, but that is not a federal agency; the acronym stands for the American Association of State Highway and Transportation Officials. AASHTO has no real enforcement authority over the various state departments of transportation, but nevertheless the states do (usually) defer to AASHTO's decisions regarding US routes. AASHTO once published a route log with distances, but those mileages were derived from the totals that were provided to them by the individual state DOTs... and the latest version was revised in 1989! Around 2010 they expressed a desire to correct/update the log, but it appears the state DOTs showed very little interest, and I am under the impression that AASHTO has tabled the idea for now.
Many US routes run through a half-dozen or more states. If just one of those states provided an inaccurate figure to AASHTO (or they have changed the alignment of a US route since 1989), then that means the published mileage for the entire route is wrong as well. I suspect those kinds of things have actually happened multiple times with just about every US route, and as a result one can find many different figures for the length of any given US highway. In light of all this, one should view the data on AASHTO's route log with cautious skepticism. And I frequently find that even state DOTs have misleading or inaccurate information for their own routes. Therefore Wikipedia is often incorrect as well, since editors are required to cite "official" sources.
It is almost always of one of the longer US routes about which people want to know the mileage and/or the midway-point. And that makes sense: I mean, the halfway point along US 46? Who cares. But the halfway point of, say, US 6 or US 20? Now that's kind of interesting. But it takes quite a bit of effort to figure that out... at least the way I do it, which allows me to stay accurate within one-tenth of a mile per 100. And it is even more difficult to calculate the mileage of a route that no longer exists, or that has non-driveable interruptions in its historic alignment (like US 66). However, during the pandemic of 2020 I found myself with extra discretionary time, and I used it to measure all 291 current and former US routes. The results can be found in the header section on the page for each US highway.