What is the possible cause that make my subdomain to have higher page rank than my main domain? I can understand that subdomain and main domain are considered a different domain, but the main domain traffic has more traffic and more content while the subdomain only has 3% of the main traffic.
closed as primarily opinion-based by dan♦, Zistoloen, John Conde♦ Aug 9 '13 at 11:41
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Page Rank is calculated on the value of incoming links to the URL; amount of traffic and content does not factor into this calculation. So there must be more 'quality' websites that are linking to your sub domain than your main domain.
If you have Google webmaster tools set up on the main domain and on the sub domain you can see a selection of the backlinks Google is seeing for each domain. Looking thought these you might be able to get a better understanding on who is linking your site.
I have a client and friend who owns a franchise; until recently they were on a subdomain set up.
I believe their corporate office chose sub-domains (years ago) when search engines truly viewed subdomains as separate sites and therefore you could weasel your way into having multiple results from a domain on an SERP.
At least with Google, that is rarely the case anymore (as per Matt Cutts in one of his Google Webmasters YouTube videos).
Unfortunately, corporate never did any sort of SEO (many pages with duplicate titles and meta descriptions) but my friends did (with my help). We reviewed analytics and webmaster tools and made appropriate changes.
Truth be told, I think this pissed off their corporate office, not because we outranked them, but because suddenly they were held accountable and felt that my friends were 'off brand' due to making needed content changes.
Anyway, their local franchise out ranked the main corporate site on a number on terms that didn't include a geographic reference because we did a bit of on-page and off-page optimization.
The quality, and less so the quantity, of links (and content) matter. If your subdomain outranks your primary domain I'd say you might want to consider what you have done differently. It might be as simple as relevance to queries you can rank for but it might also be related to penalties for low quality links or low quality content.