Maybe US operates slightly differently than DS in that respect?
Certainly there are a couple of factors that would look different for US:
- UPBO means that the chosen transmit power alters more dynamically, and SNRM values can't easily be compared between different lines
- Something (perhaps within UPBO, perhaps elsewhere) makes the bit-loading allocation for US favour the highest frequencies as much as possible.
In the latter case, my bit loading for U2 is considerably higher than U1 (6-7 bits vs 2-3 bits), with a 100m line. For my old 350m line, the bit loading in U1 was only slightly lower than U2.