Hi there Paul,
Sam from Moz's Help Team here! Sorry about the trouble - I know this can seem frustrating and confusing so let me try to explain. Overall, this is due to our method of indexing, which focuses on quality over quantity (if we're providing a summary of it).
When we collect this information for our link index it's actually expected that we won’t find every page and link because we aren’t looking for them all. When we collect this data, we’re looking specifically for the most valuable links and, rather than crawling your entire site or every site, we collect this by starting our crawler on a few highest ranking sites and letting it perform a breadth first search to see what it finds.
For each page that we crawl, we first collect each of it’s links before following these and collecting the details of each page that these link to and so on. There’s a set limit of links that we’ll crawl per page and pages that we’ll crawl per site so it’s expected that we may not follow every link on a site this way (so this means we may not always find all available backlinks/linking domains).
Open Site Explorer and the Link Analysis page of campaigns are both tied to our Mozscape index, which tends to update roughly once a month.
Just a few points on how we compile our index:
- We grab the most recent index.
- We take the top 10 billion URLs with the highest MozRank (with a fixed limit on some of the larger domains).
- We start crawling from the top down until we've crawled ~130 billion URLs
The idea here is that we're focusing on the highest-quality links we can find, coming from the most prominent pages of authoritative sites. So, while you may not see every link for a site within our index, we're aiming to report the most valuable ones available!
Most new sites and links will be indexed by our spiders and available in Mozscape and Open Site Explorer within 60 days, but some take even longer for many reasons - including the crawl-ability of sites, the number of inbound links to them, and the depth of pages in subdirectories.
You can see our most recently updated schedule here as well as some more technical metrics on our Mozscape API Updates page. You can also see when the last and next updates happened on the Open Site Explorer (OSE) homepage at any time.
Since Moz focuses on quality of links over quantity, we are always focused on the most relevant links to display to our users. It's possible that Moz's index will leave out some of the lower-quality (non-link juice providing) links out of our index because of this. So, that might explain why you may see some discrepancies with what other tools may be showing.
It's also important to note that OSE is based on link freshness and diversity, so it's possible that older inbound links that were pointing to you have no longer been indexed. Our index only caches the link data it has for around 180 days - after that, unless the tool is prompted to re-crawl those sites, links will fall out of our index and no longer be included, which is why we always recommend consistent and continuous link building.
Generally, we recommend using a wide variety of backlink tools to get the most illustrative picture of how your site's backlink profile looks. OSE and Ahrefs index differently and have different purposes. Ahrefs is good for quantity while OSE is great for finding higher quality links. Domain Authority is our own proprietary metric that is closely correlated to Google rankings, so our customers often use OSE to research influential sites to build links.
I hope this helps - let me know if you have any further questions!