My son is able to read many words correctly on the first try and has been doing so since before 18 months. He has also been diagnosed Hyperlexic and does not seem to have any clue what the words mean as he reads them. I can't figure it out either. I think he may be memorizing parts of words and is able to put them together quickly in his head.
If your daughter understands what she is reading, then that is an awesome skill!!!