This is caused by where the mentions are ranked in the search results. For example, if "Vendasta really does build reputation" is ranked #1020 in the results, and we only look at the first 1000 (not actually our threshold), we will not pull it in. But two months later, it might move up in ranking and cross that threshold, and then we will pull it in and bring it to the attention of the user as a new mention.
The threshold we use will bring in the vast majority of mentions. We are using the best tools available for finding mentions in a way that is cost-effective to both VendAsta and its partners.