Interesting, it's been like a decade since I last used windows, but I had to go and check what layouts are available, since I remember having these combinations on my layout. Apparently those combinations are provided by windows in the "Finnish and Sami" layout, which provides a number of extra letters (not just ones used by the Sami languages) through AltGr+letter combinations. I must have selected that as my layout at some point while I was still using windows, possibly for the purpose of getting easier access to letters like ÆØÕ, and just forgotten it after some time.