They are all Dell computers.
I found this on a forum that might explain:
"If you purchased your PC with Windows already installed from a large manufacturer such as Dell or HP, it’s likely that Windows was installed using an OEM key instead of the actual key for your PC. This saves them (and you) money because it’s much more efficient for these large companies to install Windows once and then clone the drive."
On another site, someone said this:
"Don't forget that the Key you extract from the registry on an OEM install is unlikely to match the Key on the CoA even though the install may be genuine as it will be the SLP activation Key for that OEM."
An explanation of an SLP installation (System Locked Preinstallation) is here: http://en.wikipedia.org/wiki/System_Locked_Preinstallation
What I don't know is if either key can be used to reinstall the OS, if needed.
Any thoughts?
Jono