sdb2 will not show up in df commands because it is a "Linux LVM
" partition type and is a part of a VG (VolGroup00) out of which LVs (LogVol00 and LogVol01) are created and mounted. In your case sdb2 is nothing but the root partition (/) + default swap.
If you use
your numbers will tally up closer.
If you compare that to sdb1, this is a "Linux
" partition type and the partition will simply appear as /dev/sdb1 when you run df -h.