The slight difference is actually explained on this site...
http://www.cnde.iastate.edu/cnde_new...the_tapper.htm
"When the composites group in CNDE took on the job of modernizing the old practice of coin tap four years ago, they had no idea that they would one day use the instrument they developed to map out damages on a rudder of an Airbus A300 failed by its composite tail."
"...a tap test samples the local mechanical property of the structure, specifically the contact stiffness. This is quite different from a global test such as listening to the ?ring? of a railroad wheel after it is struck with a hammer.."
Continues..