What are the counts on the left on a search result page?
On every search result page Topsy provides you with the count on the left which is the number of search results for that period. It is a count of tweets, uniqued by links or retweets: i.e. multiple retweets are only counted once; multiple tweets for a single url are only counted once.
As twitter is growing, and our counts are cumulative, the number of tweets for a topic can to be higher if you check it on any given day compared to the previous day, especially if you’re looking at the number over the past month.