Subdomain vs Root Domain Metrics

[Optimise]

My site has everything in the root - test.com.   www.test.com is redirected to test.com and all content is in the root; there are no subdomains.

So I'm completely confused as to how the mozrank and moztrust are different.  They have the same number of links yet:

Subdomain:

MozRank - 3.72
MozTrust - 3.23

Root Domain:

MozRank - 3.05
MozTrust - 2.79

This seems consistent with all my competitors too so just wondered if someone could explain why the root domain is usually/often lower?  Surely the root domain numbers should always be equal to or higher than the subdomain?

[Optimise]

I think I understand the basic idea of mozRank and mozTrust, just confused as to how it's possible for a subdomain to have a lower score.  I guess only by having multiple sub domains can your overall root domain rank as high or higher than your subdomain...

I have set the preferred domain to the non www. site and all www. are 301'd to non www.  In sitexplorer if I view the www. then there is one followed link that is 301'd to the non www. site.  So that's why I'd have thought they'd be the same.

[DarinPirkey]

mozRank represents a link popularity score.  So it could be a situation where people are linking to your site differently.  (http://www.seomoz.org/learn-seo/mozrank)

mozTrust is more complicated because it has many factors.  Here is a WBF on it by Rand (http://www.seomoz.org/blog/whiteboard-friday-domain-trust-authority)

That being said, have you set your preferred domain in Google WMT?

Have you used OpenSiteExplorer.org to see both link profiles (www vs non www)