All Windows Workstations includes all managed non-Server Windows systems known to the NS.
All Computers includes all managed computers, regardless of OS, known to the NS.
Resources > Default > Computer includes all managed and unmanaged computers.
You are correct -- Default should not show the lowest number. Are you sure this is the case? If so, I recommend sharing the SQL used for your collections -- possibly someone has written a custom collection and the SQL behind it is not good.