[CentOS-mirror] Why 32bit packages in 64bit repositories?

Just curious to know why there are 32 bit packages in centos 64 bit 
repositories?

One of my clients were installing curl packages for his 64 bit machine via 
yum and realized it tried to install both 32 and 64 packages.  At first I 
thought I had my mrepo configuration on my mrepo server was all wrong but it 
turns out it's fine, and it's really the centos.org mirror that has this 
issue - http://mirror.centos.org/centos-5/5.2/os/x86_64/CentOS/ (look at all 
the 32 bit packages!!)

Can someone explain this logic to me?  Sorry I'm a newbie.  Thank you.

John