Here's the answer I received. I have to say, I still don't get it. On the site I manage we have internal links in the body of many articles, side widgets, footer, HTML sitemap, Related Content, and so on. Yet MOZ has us -- constantly -- at 66 internal links.

Here's the MOZ answer I received to my question about this:

I think it may help to understand a bit more about how our index is built. Our index doesn't work like a direct crawler that you'll see with Screamingfrog, or Moz Pro Site Crawl. Direct site crawlers go right to your site and crawl from page to page using links. This would give you an accurate picture of your site at the time it's crawled.

Our index is built by crawling and indexing high authority sites and pages and following links internally and externally. If we find a link to a site we may not go on to crawl and index all the other pages on that site. It's likely we won't have a complete map of all your pages and internal links.

The index and Link Explorer tool are designed to help you discover prospects to build your backlink profile, rather than give you a complete picture of your internal links. The higher your pages authority to more likely they are to be crawled and indexed.

You can learn more about our index here https://moz.com/help/guides/link-explorer/how-we-index

I'm curious about this too, but the answer above is to a whole other question. The question wasn't about backlinks, it was about internal links counted by Moz.