Instagram provides a way to get embed codes for all photos so they can be added to websites. When you get an embed code from a user's page, it includes their IG username. But if you embed a photo from Instagram's explore page, they credit it as "Instagram" and do not include the photographer's user name at all.
When Instagram first introduced embedding they stated that "...everyone will clearly see that your content belongs to you. Your embedded photo or video appears with your Instagram username...":
"...As always, you own your photos and videos, and we want to make sure that’s understood no matter where your content appears. Whether you want to embed your video on your blog or a friend wants to feature your photo on a website, everyone will clearly see that your content belongs to you. Your embedded photo or video appears with your Instagram username, and clicking on the Instagram logo will take people to your page on Instagram.com where they can discover more of your photos and videos..."
Embedded from Instagram's user page. Notice the photographer's Instagram username is attributed:
Embedded from Instagram's Explore page. Photographer's username is not attributed: