I don't know the possible causes or the whys... but until you can figure out what the bigger issue may be here's a few hints for helping her read and not just memorize books.
go back to the word level and work just on single words
make up flashcards that are color coded or shaped or whatever to tell them apart, say on the green ones put "nouns" then on the pink ones put "verbs" then on yellow ones put "adjatives"... (this is your prep work she's not necessarly learning nouns, verbs and adjatives....) then what you will do is draw a card radomly from each pile, put them together and make a real sentence that has some sort of meaning to it.
cover pictures in books or print out stories/nursery rhymes with just the words, no visual clues.
mix up sentences on pages, asking her to read the 1st the last or just the middle one.
Hope this helps some, at least it will bump up the thread a little so someone with more insight might see it and respond.