What is the difference between template_preprocess_search_result() and template_preprocess_search_results()?
template_preprocess_search_result is to process variables for search-result.tpl.php which will render output for each of the search result independently.

template_preprocess_search_results is to process variables for search-results.tpl.php which will render output after each of the search result is already rendered through search-result.tpl.php

For example, if there are 4 search results. for each of the results search-result.tpl.php will be used. and at the end search-results.tpl.php will be used. Ending up using search-result.tpl.php for 4 times for each of the results and search-results.tpl.php only once at the end.

Hope that is clear.