Without knowing anything about this product, I'm going to say that both "Deimos no Hanayome" and "Akuma no Hanayome" are
equally valid titles. While Akuma no Hanayome may or may not be the "officially correct" title, it is definitely not an incorrect reading - quite the contrary, most people would have assumed it to be so had the "furigana" not existed and there would be no possible way a normal person would have derived "Deimos" from the Kanji. Accordingly, "Bride of Deimos" is a perfectly accurate English translation.
To put it another way, while the main function of furigana is to indicate the reading of a Kanji or Kanjis, that is not its only purpose. It does two things here: to specify the name of this particular demon (i.e. Deimos), and to communicate to the reader of the existence of an alternate title (i.e. Deimos no Hanayome). The first function (adding more detail or "flavour") is commonly seen in manga. An example could be something like 貴方(or even just あなた) and its "furigana" would be [protagonist's name].
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Another use is to write the kanji for something which had been previously referenced, but write furigana for sore (それ) or are (あれ), meaning "that". This means that the actual word used was "that", but the kanji clarify for the reader what "that" refers to.
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A similar practice is used in native fiction to clarify extended meanings. For example, in a work of science fiction, some astronaut could use the word ふるさと, furusato, meaning "my hometown", when referring to planet Earth. To clarify that for the reader, the word furusato (hometown) might be written in hiragana over the kanji for chikyuu (Earth).
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http://en.wikipedia.org/wiki/Furigana
The topics and ideas expressed in this post can also be applied to other albums on the website in which the Kanji's standard readings do not match the "furigana" paired with it.