Take a look at the above example.
This particular site, when analyzed in Site Explorer, shows "No Data" for both backlinks and referring domains.
How then can this domain have a UR of 0.9 and a DR of 54 without any data on backlinks?
Let's plug in the non-www version:
It turns out that this particular domain has 21K backlinks and 798 referring domain going to the non-www version.
This explains why the domain has a value for UR/DR.
Why are they different?
Mozilla has a great article on why. I'll encourage you to read it.
Some tips:
The next time you're analyzing your site (or another site) in Site Explorer, make sure you check both www and non-www versions to see if there are any discrepancies.
If you still see "No Data" after checking both cases, please contact support and we'll take a look at it for you.