Liturgical confusion

Star and I made a curious discovery at Mass this morning. Our parish uses missals from a series that divides the liturgical cycle into six parts - years 1, 2 and 3, each with a part one and a part two volume. Star was given the standard year 2, part 1 missal, but couldn't find her place in the readings. We noticed her readings didn't match up with mine and had a Gospel reading from Luke. When I checked the rest of the book, all the rest were also from Luke. Apparently we had a year 3, part 1 book disguised in a year 2, part 1 cover. What puzzles me is that the book looked fairly well worn and had obviously been in use some time. Had nobody else ever noticed? (And yes, I did hand the book in to our parish priest to take it out of circulation!)