Query caching

Actually some search engines cache SERPs for frequent searches and display the cached SERP instead of a live SERP to increase the performance of the search engine. The search engine updates the SERPs periodically to account for new pages, and possibly to modify the rankings of pages in the SERP.
SERP refreshing can take several days or weeks which can occasionally cause results to be inaccurate or out of date, and new sites and pages to be completely absent.