Are you running AD import on a regular basis? If so, it may be overwriting the value. We had this problem frequently with NS6, AD Import, and Server 2003 x64 where AD Import would reset the "System Type" column from Win64 to Win32. You can increase your "send basic inventory" schedule (we send it daily) which may help.
Does the same issue persist if you "send basic inventory" from one of the machines, then check the AC_IDentification record in Resource Manager for the machine? You may also want to ensure that the AeXBasicInventory.dll in your Inventory Solution package (\\nsserver\nscap\bin\win32\x86\Inventory Solution\) is the 6.0.2416 version; this is a known issue with all versions of Inventory Solution. Also, you mentioned you are using NS 6.0 R13; but have you upgraded the other Solutions to the proper version to support Win7? Take a look at KB article DOC2031 for more details.