A former New York Times reporter pointed out in an email that “the Sunday Styles is being printed on higher quality paper — acid-free or acid reduced as well as white? Presumably advertisers wanted. Cost?” I noticed the same, and asked Times spokesperson Eileen Murphy about it. She responded:
The switch to a higher paper quality in Sunday Styles happened in mid-December. …We won’t talk about cost, but the decision was made because we felt that the type of coverage we carry in the section (fashion shows, other highly visual profiles, etc) and the type of advertising (only section where we run billboard ads) warranted it. It was a decision to serve readers as well as advertisers.