In Google's own words they only preload the top result if they are confident it is what you are looking for. If they pull it off there will be negligible amount of unused preloaded pages.
Shouldn't the top result inherently represent what Google thinks is what I want? Or is there some secondary "confidence" component to the presumed efficacy of a top result?
I'd presume they could look at the difference between their confidence (relevance?) in the first and second results. If the difference is more than some threshold, bam preload the first.