"The length of pieces is very different."
Translation:La longueur des morceaux est très différente.
My thought too, but it could be meaningful--i.e. it could mean something other than the simplest assumption. If Jones has a collection of 20cm pieces of string; and Smith has a collection of 30cm pieces of string, then I might say: Smith's collection is just like Jones's. And you'll correct me by saying "Not really. The length of the pieces is very different." --That is the length (singular) that all Smith's pieces have is (not "are") very different from the length that all Jones's have.
But more likely it's an error.