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.