Depending on your NS Purging Maintenance settings, computers could be configured be retired in instead of deleted, the sql statement "isnull(i.Name,'Active') as [Status]" will report the retired status of the computer if your NS Purging Maintenace Settings is configured to be retired.
this is useful if you want to find out if computers imported from the MS AD Import connector that are in a retired status are still in your AD.
the ResourceAssociationTypeGuid = '3028166F-C0D6-41d8-9CB7-F64852E0FD01' is for asset
The [# of AD Imports] represents the number of AD Imports that have been ran on a computer resource. The Evt_Directory_Import_Status table is one to many table, meaning a resource will be in this table many times depending on the number of times it has been imported. The Computers discovered yesterday where discovered by the AD import connector and that is why they have a number in the [# of AD Imports]. The field "last AD Import" should match the time of your configured MS AD Import rule for computers.
The descrepancies between the "Days Created" and the "[# of AD Imports]" could be caused a number of factors. The Days Created will represent the date the computer was created in Altiris. The methods to add a computer resource in Altiris, include AD import through the connector, Import through Domain Membership/Wins, network discovery, and installing the symantec management agent on a computers (oh yeah if you have the asset managment suite you can manually create an computer asset also). So I would look at the CreatedDate and Days Created values as the date Altiris created the computer resource and the [# of AD Imports] as the number of the times a AD Import has been performed on that computers resource. So also another secnario would be if you created another computer ad import rule, lets say ten or more so days ago that could reflect why the is a difference, and then there is also the possiblity that your NS is configured to purge records in this table based on your NS Purging Maintenance settings.