scrapy:为什么有些 Crawled (200)的没有 Scraped 下来

最近用 scrapy 框架做爬虫，前几天的数据一直都没问题，这几天爬取的数据明显减少了；应该不是代码的原因，看了下日志，有些 URL 没有抓下来，怎么回事？

2019-04-01 00:00:03 [scrapy.core.engine] DEBUG: Crawled (200) <GET https://www.yellowpages.com/search?search_terms=Tires&geo_location_terms=Dundalk%2C+MD&page=4> (referer: https://www.yellowpages.com/search?search_terms=Tires&geo_location_terms=Dundalk%2C+MD)
2019-04-01 00:00:06 [scrapy.core.engine] DEBUG: Crawled (200) <GET https://www.yellowpages.com/search?search_terms=Tires&geo_location_terms=Dundalk%2C+MD&page=3> (referer: https://www.yellowpages.com/search?search_terms=Tires&geo_location_terms=Dundalk%2C+MD)
2019-04-01 00:00:06 [scrapy.core.scraper] DEBUG: Scraped from <200 https://www.yellowpages.com/search?search_terms=Tires&geo_location_terms=Dundalk%2C+MD&page=3>

如上，page=3 的就 scraped 下来了，而 page=4 的这个只是 crawled 了并没有 scraped，这是为什么，存在好多这样的情况。