Crawling (Drupal 7 site) but links are indexed as (without the www). Why?

I’m testing Swiftype for a Drupal 7 site, and I’m having a problem where even though I’m asking to crawl, some of the pages being indexed are showing as their URL in the search results. The protocol is wrong (http instead of https) and so is the domain prefix. Why is it doing so and what should I change to fix it? Is this a Drupal thing?

The site is only using a robots.txt file to restrict the crawling to certain areas, and does not have a sitemap.xml file.