My son and daughter both see a word and since it looks like another word they know, their brain assumes it's that word and then skips merrily along.
Part of it is that their brains are scanning ahead of where they are reading and predicting what they think it will say. For my son, if I ask him to read aloud, it's like his brain read it and paraphrased it before it comes out his mouth.
What I do is try to get him to slow down his reading. Have her read aloud with you and see if you can pick up on any cues that will let you know where she's going astray. I used to read chapters with my daughter then ask her about what happened. You could also read ahead and leave a sticky note in there at the end of the chapter if she's reading on her own without you. Sometimes you can find study guides for popular books online.
