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This can be pretty easy to do: they could do few 301 redirect from domains with many inbound links they could place sitewide footer links on websites with many pages indexed I don't believe it's a blackhat in 100% but I'm 100% it's not good in long term. Also DA increased so links come from high DA domains too. However high DA doesn't guarantee links are good for seo and always safe.

That's a fairly thorough redirect and has a pretty high likelihood of not delivering as much link juice compared to a standard, dead simple link.  Still, it's a nice page and those are well-known brands, so appearing there would have positive branding benefit for human eyeballs.

The result set for this query is very interesting, lots of diversity in query intent: you have have definitions, brands who specialize in pos systems, reviews of pos systems, and brands who have pos systems, but whose primary focus is not the pos system itself. While the Shopkeep page you linked to does not have the most links or highest authority of the pages in the first page of results, it is the strongest domain from in the 'brands who specialize in pos systems' category noted above. My hypothesis is that Google believes the primary user intent for the query is to purchase a pos system, therefore, is giving favor to the most credible brand whose business appears to focus primarily on pos systems. If you look broader than just the page in OSE, you'll see that 'shopkeep pos' is the second-most used anchor text for inbound links to the whole site, representing a total of 29,145 links from 91 different domains. This would support the above hypothesis that Google finds the brand to be highly credible for pos systems because it is so often used alongside the brand name. Hope that sparks some ideas for further research! -Trung