I was riding my bicycle home after picking up sushi for the kids, and it all became clear. The purpose of the USHCN adjustments is to hide the UHI effects in the GHCN data set.
More than half of the GHCN US stations are at airports – and are no doubt strongly affected by UHI effects. Thus the strong warming trend in the GHCN US daily data, which is not seen in the USHCN daily data.
In order to make the USHCN data match the UHI tainted GHCN data, they had to implement some huge bogus upwards USHCN adjustments.
The honest thing would have been to downwards adjust the GHCN data for UHI, but that just isn’t how climate science works.