[CentOS] /dev/sd and /dev/mapper

I've done a little research, and can't really find out why it happens, 
but on one server I have built, I get the old fashion /dev/sda1, 
/dev/sda2 type partitions and on another server I get the /dev/mapper 
style partitions. Both machines are hardware raided (before the install) 
and both machines are Centos 5.5 64 bit, using the same set of CDs to 
install from.

Can someone please explain why this happens? I'm not sure it matters, so 
this is just a curious bother. The machines are different brands with 
different raid controllers. Both were partitoned using the "custom 
layout" when installed. I'm thinking I must have overlooked a checkbox 
or something during the install.

This is not a big deal, not a problem, it's just an itch in my brain.

Thanks for any explanation.

steve campbell