Twitter search results are divided into sections. On the left, you can toggle between these categories: Tweets, People, Videos and Images.
These results are determined by Twitter’s search algorithms, as are the “top” tweet results.
In Twitter’s words, “We’ve built an algorithm that finds the tweets that have caught the attention of other users. Top Tweets will refresh automatically and are surfaced for popularly retweeted subjects based on this algorithm. We do not hand-select Top Tweets.”
For this reason, if you’re looking for an at-a-glance look at a topic, Top Tweets is the way to go. If you’re carrying out a more stringent search, be sure to click “All.”