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Site B uses a domain name that hadn't been used before 2013, the site is barely more than a year old. Plenty of inbound links using target keywords as anchor text, but from fewer domains.
"From fewer domains" not completely clear, and could perhaps mean simply that...
a) it had fewer inbounds, or...
b) that it had more, not less, repetition of anchor text per linking domain. As a spam signal, this should work against these links.
So even though the new site's domain was only registered last year, it's at the top of the SERPs for a key phrase used consistently as anchor text on thousands of pages of a handful of websites that the owner of Site B seems to manage. And those websites plus their content are COMPLETELY unrelated to the anchor text keywords, and aren't remotely related/relevant to Site B's products/services.
That's the type of backlink profile which is generally thought to trigger a Penguin demotion. But the site might have been too young at the last Penguin rollout (Oct. 4, 2013) to get penalized then.
But it could be headed for a Penguin demotion at the next roll-out
hose that took Google's advice, and followed their guidelines, have been slapped with a penguin penalty. Therefore new sites are free to rise above those small businesses that have been serving members of their communities and customers across the globe for years.