When you have a plank, post or beam that has stood in place in an unheated barn for 400 years, it has been subject to the vagaries and every permutation of hot & cold, damp & dry.

These cycles bring out the best and worst of the wood. Obviously you are recovering old wood in a barn for other reasons as well, but in the ladder scenario, they want top notch wood that will last. Since SFFD has ladders that have been in active service since 1919, they obviously have that down.