Ahrefs Rank is the list of all the websites on the web ordered by the size and quality of their backlink profile (basically, by their Domain Rating). 

Even if your website didn’t lose any backlinks, other websites could gain them along the way. As a result, they would get a lift and outrun you.

Pay attention that Ahrefs Rank can be very unstable after a certain threshold.

For example, if your website’s AR is more than a million, it can easily drop by a few million at once. 

But when you get to thousands, AR becomes much more settled.