You are supposed to search for Twitter as a test. Tweetbot doesn't make much sense as a search term, at least for finding other Twitter clients. I think the search results are actually quite great, considering. (The calculator is an app by the makers of Tweetbot. I actually had that same use case before: I knew that some developer of some app made some other great app, but I didn't know its name – so I would search for the app I knew, go to it and then the dev’s page and then to the app I actually want. So showing other apps of that same dev in a targeted search for the most popular by far app from that dev makes total sense to me.)