The idea that reducing bulk of unsold products continues to float around as an alleged method of improving zRank, but that doesn't mean it's sure-fire.
I can confidently say that the number of unique items sold compared to total items available impacts zRank. Hidden products aren't counted so hiding seasonal items that haven't sold before could raise you zRank temporarily. But are you hiding 50 out of 500 or 50 out of 5,000? That's a big mathematical difference, obviously the more products you have listed the harder it will be to achieve & maintain a high % of sold-to-unsold. No way of knowing what level of percent change triggers a rank change, I just know that it does and with a very small store like my alternate PB store, it's easy to see that cause & effect whereas with a typically large store it would be hard to relate a zRank change to any one specific thing.
* Not recommending hiding/unhiding, just wanted to note that sold-to-unsold is one of the factors that goes into zRank and Hidden items don't count in that.
