Kind of a weird etymology/portmanteau question:

I was looking at something online, and a title of a book was 神淵の追想曲

The English translation provided was: Reminiscence of the Divine Abyss. I did the study to learn that Reminiscent Song might be a better translation for 追想曲, and this is fitting with the naming convention of other works in the series.

But then I kind of was blindsided by the Kanji lookups on 神淵. I'm seeing the On readings for the kanji as "Kami Fuchi" for [divine] and [pool or abyss] respectively, so I get the English translation, but the romanized title is listed as "Kabuchi." I'm aware this isn't a standard word, so I'm calling it a portmanteau (please correct me if there is a better term for that), but I'm confused how a native reader knows to drop the mi from kami, and shift the fu to bu in the blend?

Thank you for any explanations you have.

by corrinmana