You are correct. Dell's original OS DVD is basically a pre-activated Microsoft OS DVD. Therefore, it does not have the RAID controller drivers slipstreamed. Those drivers are included in the Factory Restore partition. When installing from the DVD, you either have to have the Matrix drivers on CD or USB or you have to change the BIOS Setup hard drive controller from RAID to AHCI, install Windows, install the Matrix drivers and then change the BIOS Setup back to RAID. That is not necessary when installing from the Factory Restore partition.
Glad you got Windows 7 working.