SATA RAID on VIA 8237

Hi! I somehow managed to install on Serial ATA on MSI K8T motherboard with VIA 8237. But I have some questions. At the install time, kernel sees two drives, even If I created from RAID menu RAID array for RAID 1 for mirroring. So I have installed on /dev/sda. Now I am wondering why there are two drives, that kernel shows. Shouldn't be only array seen? If I install it on /dev/sda means this also, that since it is array, /dev/sdb is mirror from /dev/sda? Or I am missing sometning? MIlan