[CentOS] Ordering of disks in laptop

I have a Lenovo laptop where I recently replaced the harddisk with a 
Intel 530 240 Gb SSD (SATA III) on which I then installed Centos 6.5 
which runs just fine albeit at SATA II speed. I then replaced the 
DVD-drive in the Ultrabay with an identical Intel SSD, the 
Ultrabay-adapter apparently uses a PATA-SATA bridge so this drive would 
only run at PATA speed.

Being a Centos newbie, I am not sure why Centos calls the first SSD sdb 
and the second one in the Ultrabay sda? It would seem to me that the 
first one, the one replacing the original harddisk, should be the main 
one and be called sda but clearly there is something I don't understand. 
In the BIOS the SSD replacing the harddisk has a higher boot order 
priority then the SSD replacing the DVD-drive.

Suggestions?

Thanks.