Question regarding signal code books and signal corps

[gary]

The semi-ironclad Keokuk was sunk in Charleston Harbor during Admiral DuPont's failed attack. Besides salvaging its guns, the Confederates also salvaged the signal code book. However, they were unable to understand it and used deception. Luckily, a Union signal officer was held prisoner in Charleston and a Confederate donned the blue coat of the yankees and misled the Union PoW into believing that he was a recently appointed signal officer and that he was going to be exchanged soon. He explained that he needed to learn the signals and thus was taught them by his fellow prisoner. Upon being exchanged, he promptly taught his fellow Confederates and thus the Confederates at Charleston were able to read the signals between the Federal Navy & Army. (See Col. Charles H. Olmstead's “Reminiscences of Service in Charleston Harbor.” SHSP, Vol. XI, April-May, 1883, Nos 4-5, 161.)

So, here's my question. If one has the signal code book, why the deception? Shouldn't one be able to understand it straight from the book or is there something more elaborate here? Thanks in advance.