yum bandwidth throttle not throttling

I have recently installed RHEL 6.1. I have modified /etc/yum.conf to throttle the bandwidth that yum utilizes when downloading updates. I am trying to limit yum to 20 kB/s, but it uses up all available bandwidth on our T1. Here is my /etc/yum.conf file:

[main]
cachedir=/var/cache/yum/$basearch/$releasever
keepcache=0
debuglevel=2
logfile=/var/log/yum.log
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1
installonly_limit=3
throttle=20k
exclude=kernel*

yum continues to use 160+ kB/s. Am I missing something? Any ideas?