305 GB hard drive reduced to 266 GB (why?)

[Peter]

I am installing a new 6.2 STABLE system and I am troubled by the amount 
of available disk space I'm getting on one of my IDE devices:

# df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
...
...
/dev/ad1a      289G    4.0K    266G     0%    /ad1

This drive is 320 GB by the manufacturer and thus I should theoretically 
have 305 GBs.  This is confirmed by dmesg:

ad1: 305244MB <WDC WD3200JB-00KFA0 08.05J08> at ata0-slave UDMA33

Steps:

# fdisk -i /dev/ad1
# bsdlabel -w /dev/ad1
# newfs /dev/ad1a
# mount /dev/ad1a /ad1

Info:

# bsdlabel /dev/ad1
# /dev/ad1:
8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
  a: 625140319       16    4.2BSD     2048 16384 28552
  c: 625140335        0    unused        0     0

Thanks in advance for any pointers,

PM