The usual answer, my friend. Music tests (done routinely for current-based formats, less often for gold-based) will often include songs not in the present rotation. Usually, any such "new" title will be tested because of its exposure elsewhere which may make it viable.
A recent example is the 1989 song "The Best" by Tina Turner, which had a very low amount of airplay, eclipsed mostly by "What's Love Got To Do With It" and "We Don't Need Another Hero" (with some play for "Better Be Good To Me" in the mix ... mostly in smaller markets). Then it was featured in the fifth and sixth episodes of last year's television biography of Formula One race driver Ayrton Senna (who had made a surprise appearance on stage with Turner in 1993 at the Australian Grand Prix). Apparently it resonated well with test audiences because it now gets better play than any other Tina Turner song other than "What's Love ..."
But for the most part, there is very little actual turnover in gold-based libraries. You can't "refresh" in those because they aren't making new gold titles. The most that happens is songs may increase in popularity and be played more often, or they may lose ground and gradually fall off the active playlist.
Your use of the word "occasionally" in your question pretty much tells the story.