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.
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.