Agents in Windows read and send the incorrect registry data

I want OCS Server to show the default printer in Windows 7. That depends on the user who has logged on in Window. OCS is configured to obtain this data from the Windows Registry:

REGTREE: HKEY_CURRENT_USER
REGKEY: Software\Microsoft\Windows NT\CurrentVersion\Windows
REGVALUE: Device

I don't know why some agents fail and reads this other key:
HKEY_USERS\.DEFAULT\Software\Microsoft\Windows NT\CurrentVersion\Windows

The key .DEFAULT (or S-1-5-18) corresponds to Local System user. If I change the account of the "OCS Inventory Service" to a normal account in services.msc, tab "Log On", then the agent sends the correct data.

Why do some computers send the correct data but others not?