I signed up for three categories at BookBub (Young Adult, Thriller, Science Fiction) so I could get the daily newsletter and watch the ads they serve to me. As an experiment, I held off opening the ad until later in the day to see if it serves a different set of ads - under the assumption that the early ads expend their budgets and then let the lower budget ads come through later in the day. My thought process is that the late-in-the-day ads are the ads with lower budgets. I was expecting lower-quality books/ads to serve later in the day, but so far it kinda appears that the reverse is true. The books/ads aren't necessarily worse, but they hold their own against the ads that serve if I open the email in the morning.