If it helps to remember, there is no Japanese pronunciation of the sound "fo", and I don't think "futon" has any other spelling even in English. So it has to be "ふとん".
The dictionary is following the spelling rules in order, so it sees that spelling and thinks you typed in "ふ" "お" "う" "とん" in order. Since there is no such word with that exact spelling, it breaks it down as if it was a sentence or a really long combination of words. The first part is that it sees "fo" as "f(u)o" followed by the "う" as a separate letter.
The big long lists of ふ and う include the alternate Chinese loanwords as explained. It also just so happens that many kanji have just the one syllable, so the dictionary creates a possible list of the sounds that you provided based on whatever you may be looking for. That's just what they sound like.
