df confused over disk usage

[Nik Makepeace <nik@nikmakepeace.org>]

Hi guys,

I am not sure if this is the right place, but if I'm out please point me in 
the correct direction.

df is reporting my disk size and usage a little oddly. The following is the 
df output.  Note the identical /var/local, /var/log and /var sizes, and the 
identical /usr and /usr/local size and use% values

Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/sdb5               474443     26238    423708   6% /
/dev/sda5               964500    336036    579468  37% /var/local
/dev/sda6              1438888    527988    837808  39% /usr/local
/dev/sdb1                23302      3124     18975  14% /boot
/dev/sdb6              1438888    527988    837808  39% /usr
/dev/sdb8               964500    336036    579468  37% /var/log
/dev/sdb9               280005        15    265534   0% /tmp
/dev/sdb10              964500    336036    579468  37% /var
/dev/sdb11              766848    508256    219636  70% /home

This is odd because, just like it df reports, they are on different 
devices, in some cases different disks.  Here is what fdisk reports:

Device Boot    Start       End    Blocks   Id  System
/dev/sda1             1       256   2056288+   5  Extended
/dev/sda5             1       122    979902   83  Linux
/dev/sda6           123       256   1076323+  83  Linux
/dev/sdb1   *         1         3     24066   83  Linux
/dev/sdb2             4       555   4433940    5  Extended
/dev/sdb5             4        64    489951   83  Linux
/dev/sdb6            65       246   1461883+  83  Linux
/dev/sdb7           247       270    192748+  82  Linux swap
/dev/sdb8           271       300    240943+  83  Linux
/dev/sdb9           301       336    289138+  83  Linux
/dev/sdb10          337       458    979933+  83  Linux
/dev/sdb11          459       555    779121   83  Linux

As you can see, sda5, sdb8 and sdb10 are all different sizes, as are sda6 
and sdb6.

Anyone know why df is getting it wrong?

Nik