Yeah this had always confused me, because Ashkenaz was the son of Japheth, not Shem (Semite) according to Genesis 10.
Ashkenazis aren’t called that because they are descendants of Ashkenaz, but because they became noticeably different residing in Ashkenaz (aka Germany or generic Europe). They were cut off from Middle Eastern and North African Jews because of Islam.
Japheth was the progenitor of Greeks/Europeans.
I believe that even Sarah's mother may well have been out of Japheth. Abraham had both mother and father who were Shemites. Sarah had the same father as he, but a different mother. I think that right at the beginning the strain of Japheth was strong in Abraham's Shemite strain, as the wife.
Remember that Sarah and Rebekkah were considered so beautiful that kings wanted them. There were many beautiful women you can be sure. What made them different? It is likely their lightness. Even today many, many countries that are darker skinned see their lightest skin women as being the most beautiful.
Remember that the two girls who were Jacobs wives were taken right back out of that strain. Rachel's brother's name was Laban. Names meant something then. Laban means white. It may even be that their mothers were white, as whites were in that area. I think that Japheth is the bride and Shem is the groom. This is even reflected in much of God's Church.