From my own personal experiences, if it is not exposed to too much sunlight it may be due to the quality of the paper... my Singapore press of Namie Amuro's PLAY became yellow within a few months, but when I went to Japan and bought the album until now it is still as white as ever. Same goes for your case I guess
EDIT: My SG Press of A BEST 2 -WHITE- has also yellowed slightly but my Japan press like I've mentioned is still as white as ever.