The more it can crawl, the more likely it can maintain a fresh index. However, this is not the only way to measure freshness. Search engines may learn how frequently pages change or use other methods to improve freshness to maximize a smaller crawling capacity. There are various reasons why this occurs. It takes longer for them to return and gather non-submitted pages.
Freshness shows the age of listings, from best to worse case scenarios for each search engine.
Date Some search engines show the date when a web page was added. This provides a clue as to how fresh or stale the search engine's listings may be. Kudos to these search engines. The others leave you guessing about freshness. File date means that the date of the file is shown, rather than the date is was added to the index.