I was trying to read a raw copy of the Ore no Kouhai ga Konnani Kawaii Wake ga Nai and on the page shown below, there's this kanji, 定命: As seen, it has the furigana さだめ. However, when I tried looking it up on dictionaries, I only found two possible readings for said kanji: じょうみょう and ていめい. I could not find anything with regards to it being read as さだめ. So, why is the furigana different? Does it convey the same meaning despite this difference in its reading?
As described in this answer on the Japanese Language SE, there is a phenomenon known as "ateji" where the kanji and associated ruby* are not 100% aligned, either in reading or in meaning. This particular style is especially used in light novels and manga as a way to provide a double message - the ruby is the actual word spoken, but the kanji gives a clearer understanding of the desired meaning.
*A Ruby character is any smaller writing placed above the main writing, technically it's only furigana if it actually provides a guide to the reading of the kanji but that's a distinction that is frequently ignored.