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
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.