This piece is the first of a set of "Three Carol Preludes" by Richard Purvis.
A summary of his life and career can be found at http://en.wikipedia.org/wiki/Richard_Purvis, so I won't repeat it here, but two salient facts deserve mention. Early in his career, he played a nightly radio broadcast on a 7 rank Wurtlizer organ. After his appointment to Grace Cathedral, San Francisco, he developed a distinctive sound by, among other things, voicing the 93 rank Aeolian Skinner organ and regulating its tremulants in a style similar to theater organs. This does much to explain his choice of registration for these pieces.