You're not limited to targets within your own tier. Whenever you refresh and get a new screen, you get people within a certain range below and above yourself, ranked from lowest to highest rating. If you're close enough to the top of your tier at the time of refresh, that means a number of targets will be in the higher tier. If you're close to the bottom of your tier, you'll have targets in the lower tier as well. And then as soon as you get into a new tier, up or down, it will auto-refresh and give you new targets (which can be frustrating if you had a few easy targets left on your previous screen).