You did the redirects. Most people do, and most people get the ones they can see: the URLs on the old sitemap, the pages a crawl of the new site turns up, whatever Search Console will hand over.
That list has a ceiling. Search Console's export stops at 1,000 rows, the old sitemap goes when the old store does, and a crawl only finds pages that still exist. Google has been keeping score for 16 months, and it still has traffic data for URLs that appear on none of them. Most of it would only reach you secondhand, from a customer who hit a dead link or a recrawl weeks later. The report reads Google's record instead, and it runs in minutes.

