My source for the 400-series numbering convention is (ultimately, I think) the AARoads High Priority Corridor page dealing with US 400/future I-66. It is certainly not anything I have seen in official AASHTO documentation, but that doesn't necessarily mean anything since so little of it is put online where it can be viewed free of charge. I think it is a reasonable inference on the basis not just of US 400, but also US 412 being in the wrong half of the country for a fourth branch of US 12.
Don't take that seriously, it's not meant to be. We've been saying that 12.5 thing for years on these roadgeek sites. Anyway, that isn't the way it works. AASHTO doesn't assign numbers for US Routes; states request them. In the past, we've had roadgeeks communicate with state DOT's asking why they wanted both 400 and 425 for those routes, and both were answered "no real reason." If the Route Numbering committee had minutes online going that far back, maybe we could find out more on why those particular numbers were selected.
By the way, to be specific, 412 was designated in 1982, but it only ran from Dyersburg, TN to Walnut Ridge, AR. It may have gotten its number because it didn't intersect any even-numbered routes so couldn't be considered a branch of any of those, but that doesn't explain the specific number. It replaced TN 20 (actually, it's signed concurrent with 20), MO 84, and parts of AR 90, 1, and 25, so I can't see that the number came from any of those, either. It was extended further east into TN in 1984, and extended further west into OK in 1989 (incidently, the year 425 came into existence) and into NM in 1994 (incidently, the year that US 400 came into existence).
I don't think there is anything particularly strange about using 400 as the start of a series of false two-digit designations.
No, if they are arbitrary numbers then any scheme would work. I just think it's strange there's a "400-series" at all. But I don't really think there is.
BTW, I am not sure Granada is the original western terminus of US 400.
No, its original western terminus was Garden City, KS. It was extended west to Granada 2 years later, in 1996.
The original question we were addressing was not why Kansas designated it, but whether or not it still needs to exist. I still say no. On the other hand, there's no harm to it, and think of all the years of roadgeek angst we would've missed out on if it never had been.