Search result issue with Search API module
If you are not using Apache Solr in the back-end, then you will have to use the Fuzzy Search module in conjunction with the Search API module. This module provides drupal sites with a fuzzy search engine to allow for broader keyword matches including partial or misspelled keywords.

*Note: At time of writing this there is some incompatibility with Search API 7.x-1.0-rc1 and Fuzzy Search 7.x-1.x-dev (2011-May-12). To get these two working together, you will need to use the search_api 7.x-1.0-beta10 until the Fuzzy Search maintainer gets the module caught up to the current release of Search API.