I've made some tests about that. It seems that CentOS have the LVM functionality enabled by default (official doc: http://www.centos.org/docs/5/html/Cluster_Logical_Volume_Manager/)

In this case, the "df" command doesn't return the name of the physical partitions, but the name of logical partitions. Consider my virtual machine on which I've chosen all default values during the installation of CentOS.
I have one physical disk of 20 GB, according to this screenshot:
![disk centos](https://f.cloud.github.com/assets/1638158/587412/76980d3a-c965-11e2-8860-cac45f81c595.png)

The "df" command return this:
![centos df](https://f.cloud.github.com/assets/1638158/587427/e0d8589e-c965-11e2-8538-83c311f92434.png)

We can see a physical partition, the "/dev/sda1", with a size of 500 MB (approximation, 49588 KB exactly). It's only mounted as "/boot". Good.
However, as you can see, the "/" mount point which contains all the data is mapped with /dev/mapper/vg_centos-lv_root.
In reality, this **logical** partition is linked to ```/dev/dm-0```.

With this test (```if df_split[i].startswith(('/dev/s', '/dev/h')):```), we can only see physical partition, and not logical ones.
And I think it's a little complicated on a system with the "lvm" tools enabled by default :P

NB: We are also working on the disks in #52, with the UUID detection. In this case, we should use the "name" as "/dev/mapper/vg_centos-lv_root", and "display_name" as "dm-0". Moreover, it can be useful to get the "mount point" to identify more quickly the disks/partitions.