Last year I started using the Koren machzorim and I realized that the Ashkenaz editions use the wrong Shochen Ad. In most other machzorim, the section "B'fi Yisharim" forms acrostics of the names of both Yitzchak and Rivkah, while during the rest of the year, Ashkenazim have only the acrostic of Yitzchak. However, the Koren uses the latter variant for Rosh Hashannah and Yom Kippur as well. Why?
I haven't been able to locate a definitive source as to why many Nusach Ashkenaz machzorim seem to use the Nusach Sefard version on Rosh Hashannah / Yom Kippur. Art Scroll and Birnbaum does this, also, BTW. (@Noach Mi FRANKFURT- and you're not using Birnbaum??? Ah Shandeh :-) :-)
See the link @Gershon Gold referred to and view the answer, there, that refers to "Tefillah". To me, this makes sense, and this may be the reason why these machzorim used this version specifically for Yamim Nora'im, as this is the time of year where we increase our tefillot (prayers) and requests to G-d.